Aren't you? Unless he decides to give up everything to pursue dance professionally, there's no reason for you to ruin his joy of dance.

I think you were just being a little self conscious

Let the man bust a move and enjoy himself. Don't cramp his style!

As long as you weren't mad at him for dancing. Let the man have a little fun. You don't need to have rhythm for a good time to be had

Are you a sour lemon? Who cares does he have rhythm or not, you went to have fun. He was having fun, so what if others watched it seems only you had issues?!. Maybe your own insecurities, but if your fiance enjoyed it, and had an audience and he still had fun then you're the problem. Are you great in everything you do? Or do you not do some things because you're not great at them. I was born with three left legs and hips built from brick, my ears have zero rhythm acknowledgement and my voice is horrible, but I do dance on stage and sing, loud and proud. It's called being silly, or funny, or having fun. You should be so proud of him for being able to feel so free next to you, though I assume you would rather he would only be in front of audience if you approve, and you seem to put too much value of strangers opinions, and react to things not affecting you. Him dancing, and not being in rhythm isn't your problem, not even close. It's his, and he seemed to go in enjoy and that's what you do on tourism boat. What did you want him to do? Sit beside you, watch other people? Mock and complain that they are not superstars? People go and have fun, loosen up to relax, and yet every boat will always have fun that definitely feels, looks and sits like a wet blanket. Try to sing king nobody hears and dance like nobody sees.