It's a scam. Ask yourself why someone you've never met would trust you with $19,000. Also, why the 'wi-fi' on the 'cruise ship' is good enough for him to send you emails or get checks written and mailed, but is not good enough for him to do his own banking. This alleged professional guy who sells gems.
There has to be an excuse why the scammer asks someone they don't know/have never met to handle money for them, rather than do it themselves like normal people do. Being on an oil rig is a common one; a cruise ship is another. Paris is hundreds of miles inland; you can't sail a cruise ship from there to Greece. Gaping holes in the story help the scammers to filter out people who might be suspicious. You did ask him the name of the ship, and what Greek port it was headed for?
Sending a check, which you must then pass on via a non-reversible payment method is the clincher. What will happen is that you will deposit the check, which may seem to 'clear', then you transfer the funds as the guy asks, and after a shorter or longer time, your bank bounces the check so you are out all the money you sent, and if that was more than you had in your account, you now owe the bank. The bank could bounce the check because it is fake, or drawn on a hi-jacked account, or stolen, or non-valid in some other way.
The least bad thing that could happen is you lose money, but you could get in trouble with law enforcement for money laundering (what were those 'transfers'?), or involvement in fraud. Also, your bank may freeze or block your account if they see (or are told of) suspicious activity. You should report all this to your bank, and, maybe, the police. It looks better if you step forward and tell them before they come to you.
You should be aware that this 'nice guy you met on the web' (who may, in fact be a team of experienced scammers) could suddenly turn nasty if you show signs of not co-operating, and might make threats, such as saying he will turn you in to the FBI for money laundering. This is very unlikely, and your defence would be that you are a victim of deception. They may use 'the FBI' as a threat even if you are outside US jurisdiction, I heard of a Swiss guy they tried that with. Best to completely cut off this guy and block him completely. If you gave him any passwords, you should change them.
