It really depends on the size of the boat, weather conditions and so on.
As a general rule if your going out in a small boat (dinghy sized to small powerboat) then storing your kit in plastic bags is advisable.
If your going out on a charter boat then such precautions aren't needed in most cases. However make sure you have UV or polarizer filters fitted to lenses and change lenses with your back to the prevailing wind direction to prevent salt spray from entering the camera body. Change film inside the boat cabin for added safety.
Use a camera strap as a safety precaution, cameras sink quickly.
If your camera comes in to contact with salt spray wipe the camera with a damp cloth soaked in distilled water and wrung out till almost dry then dry camera with a soft dry cloth.
Clean filters as per my method described elsewhere on this site.
After any trips to the beach or out on the ocean clean your camera as described above. Salt is the worst enemy a camera has in most cases.
last time i took a fairly big boat out in deep sea.. was so excited, took a few photos then realise i am @#$@#%#$ dizzy.. puked for 3 hrs.. if i didnt play with the camera i think it would be that bad.. probably puke for 2 hrs instead