In addition to Alex B's excellent overview, I'd like to add a few more bits of advice.
First of all, one term you should know is "commercial real estate" - which is precisely what this is. There is a business element, but it is strictly (and almost entirely) intertwined to the underlying real estate, which makes this a special category of business which is generally considered simply "commercial real estate" (just like office buildings, shopping malls, etc).
All real estate and businesses value are based on alternatives - what other options are there? In appraisal, these are generally called "comparables". A professional appraiser is generally available for commercial real estate of this type. While a full, official commercial appraisal can run into the thousands, many/most (all?) appraisers are willing to sell you a simplified version of their service, which can be called a "letter of opinion" and can help you get an idea for the market price (what other similar commercial properties are running for). A loan company would strictly require this, but if you are thinking of an all cash or form of seller-financing this would technically be optional.
Your best bet is to read about some of what is involved in commercial real estate appraisal and evaluation, and you may even want to speak with commercial loan officers - even if you don't know that you want to get a loan to acquire the property! It's their job to help inform you about what is required and what they look for, so they can be a potential resource beyond your own research as well.
With this said, the only way to estimate value (and, conveniently, the best way) is to look at other properties! And by "others", I mean that you should really not consider buying absolutely anything until you've viewed at least 6-10 other options in some depth - and you probably want to double or triple that number if you are looking to make this the last big business transaction of your life. If you don't you'll be relying on little more than dumb luck to carry you through - which in this area of business, you don't want to do because the dollar amounts and liabilities involved can bankrupt you in no time flat.
With that general advice out of the way, here's a tiny nutshell version of valuation of commercial real estate.
Commercial Real Estate Valuation - In a (Not So) Tiny Nutshell
There are a few key parts involved in commercial real estate: land, improvements (buildings, docks, stuff like that), income, and wages.
Land: the value of the land is based upon what you could sell it for, as-is. That is to say - who else might want it? This alone has many important factors, such as zoning laws, the neighborhood (including your neighbors), water/utilities, pacts on the land (someone may have insisted the land not be paved into a parking lot, or really anything like that), alternative uses (could you put a golf course on it, or is the land suitable for a big building or farming?), etc. And is this in a growing area, where you might hope the value will increase over the next decade, or decrease, or basically stay flat (and possibly cause losses compared to inflation)?
Improvements: anything on the land is both an asset and a liability. It's an asset because it could add to the value of the land, but it might also reduce the land value if it interferes with alternate land uses. It's a liability, both in the legal sense and in that it requires maintenance. If you want to rent them out, especially, that means concern about any foundations involved, termites, roofs, sewage/septic tanks, utilities that are your responsibility (pipes, poles, wires), as well as any sort of ac/heating you may have, docks, and so on. These things are rarely free and absolutely can eat you alive.
Income: Ah, the best part, the constant influx of cash! But wait, is it a constant influx? Some businesses are purely seasonal (summer only, winter only), some are year-round but have peak times, and others don't really have a "peak" to speak of. If you are renting, are there issues collecting, or with people over-staying? How about damage, making a mess, getting rowdy and disturbing others?
Regardless, there is obviously some income, and this is usually the most dangerous part of the equation. I say "dangerous", because people absolutely lie like dogs on this part, all the time. It's easy to cook the books, assuming they even attempt to keep proper books in the first place! Businesses of this form often have a lot of cash business that's easy to hide (from Uncle Sam, or sometimes even the owners themselves if there are employees involved) - and fake! And some people are just shoddy bookkeepers and the info is just wrong.
But, there will clearly be some kind of yearly income involved. What does this matter? Well...how much is there? How much is tied to the owners (personal friends do business and they will leave if the ownership/management changes)?
In commercial real estate the income will be calculated for a fiscal year, and then there is something called a "multiple", which is market dependent. Let's say the whole place takes in $100k in rent a year. As part of buying this business, you are buying not just assets, but expected future income. In some commercial areas the multiple is as little as .5 to 2 - which means that the going rate is about 6-24 months worth of income, as part of the purchase price. So with 100k rent a year, that means 50k-200k of the purchase price is attributable to the income of the business. And if business is half of what you thought it would be? That means the net value of the whole enterprise decreases by 25k-100k - on top of the reduced income every year you own it!
Income provides cash flow, which should pay all the expenses (cleaning up from wind storms, replacing windows that are broken, hauling off trash, replacing a well that ran dry), and then the extra that remains is positive cash flow. If you take out a loan, then ideally the cash flow would also pay that completely so long as you don't have any big unexpected expenses in the year - and still have some left over for yourself.
Wages: Well, that money doesn't collect itself! There's sales, keeping the books, collecting the rent, performing maintenance, customer service, cleaning, paying the bills, keeping the insurance people happy, handling emergencies, and everything else involved with running the business. Someone is going to do it, and the biggest error people make here is not to put any value on their time - and to make it so they can never afford to take a vacation again! Pay yourself, and give yourself the flexibility to pay others when you can't (or don't want) to do it all yourselves.
Making Real Money
So, what's the point of all this? How do you actually make any money? In two ways: 1) selling the whole thing later, and 2) cash flow.
For 1, it's important that you not be in a situation where you are betting that in the future there will be a "person richer, and dumber, than I am now". If the current owner wanted 2 million, then 1 mil, then less, over multiple years...this suggests either he is delusional about the value of his place (and most property owners are), or that its actually hard to find a buyer for such a business. You are going to want to make sure you understand why that is, because most of the value of real estate is...well, in the real estate itself!
For 2, you need cash coming in that's considerably more than the cost of running the place. Also, cash flow can strongly change the value of the business for resale (depending on the multiple, this can make a huge difference or prevent you from selling the thing at all).
You mentioned you want to put in more cabins, more marketing/sales efforts, etc. That's great, but first, that would mean added investment beyond the purchase price. Is it legally and physically practical to add more cabins, and what is their current utilization rate? If they are only renting 10% of their current capacity, increasing capacity may be premature. This will also vary through the year, so you may find there is a problem with being sold out sometimes...but only for a small percentage of the time. Which means you'll be adding buildings only to have them used for a fraction of the year, which will be very hard to make a profit from.
If cash flow is good, ideally even being enough to cover a loan payment to help cover the purchase price (and remember that commercial real estate loans are much smaller loan-to-value ratios than in residential real estate), there is one final barrier to making money: the damn non-regular maintenance! Roofs, wells, and wooden walls all have a sad tendency to cost you nothing right up until the point they cost you $30k+ on a single day. Is there enough cash flow to make these sort of certainties (and if you plan to be there for years, they are a certainty) not put you in the poor house?
This was rather long, but I hope this overview helps you appreciate all that you'll need to look into and be cautious of during your future en-devour!
Commercial real estate is generally costly and high-risk, but also can be high reward. You'll need to compare many opportunities before you can get a "feel" for what is a good deal and what is a terrible one. You'll need to consider many factors, such as resale value and cash flow/income (which they will have to tell you and you can assume is not true, due to ignorance or malice), as well as maintenance and liabilities, before you can begin to really estimate the value of an enterprise of this sort. There are people who can help you, like appraisers and commercial brokers, but ultimately you'll need to do a lot of research and comparisons yourself to help you make a good decision.
Finally, there is no very simple method for evaluating commercial real estate value. You need a variety of information, and you must be skeptical of what you are told because of the very large sums of money involved. It is doable (lots of people do it), but you must take care and do your due diligence so you don't get bankrupted by a single bad purchase.