Most Helpful Guy
Tough spot. He is still damaged from the divorce emotionally and financially. A lot of men get their "manly" feelings from having money. Since he has no money and has to rely on you he is feeling less "manly". You have to approach this in such a way that going with you and you paying will make him manly. Things like he can pay you back with great sex, you need a man to show off, you need him to protect you while you are on vacation. You want him to be the one you have fun with. If he doesn't go, tell him you will cry. No guy wants to see that!
Most Helpful Girl
It sounds like this 'Amazing' man is also a Honest John in telling you here, dear, that he cannot afford it at this time and marvelous place with your adoring face... Sadly.
You Could still find a Girl friend to go with or even sell these online to someone who could go alone or to Someone who could find Someone with more bucks on the boat.
Unless you come up with More in store for Joe to go, I am not seeing any sea sick way today.
Good luck. xx