You may want to look into car/mobile audio speakers that are specifically designed for a marine environment. You are going to compromise the sound quality, but they will at least have a fighting chance (and won't be terribly expensive). Plus, they are installed into the bulkhead which will make them less likely to be stolen when you are out eating dinner.
My thought is to get the best marine-grade speakers you can, but plan on headphones for "serious" listening.
But I am also assuming you are going to be on something like a 30-foot sailboat. If you have Jeff Bezos money and are on a mega yacht, then you should have plenty of air-conditioned space for whatever you want.
https://youtu.be/pY2R7aOeBLU (30 foot sailboat tour)
vs
https://youtu.be/cm40e1ChozE (mega yacht tour)