What kind of boat is it? Small pleasure craft that lives on a trailer or a houseboat that lives on salt water?
Bear in mind that the smallest failure of the finish,a dent or hairline crack will act like a sponge to any moisture present. Also a salty atmosphere will actively degrade the finish even if it is inside!
Imho, unless there is almost no risk of exposure to moisture, I would try to persuade your friend to dig a bit deeper and go for 'real' timber.
Julian's hit the key issue: presumably, as he/you are even thinking about an MDF table, it's relatively large, and the table will be 'indoors'. But, frankly, if it's a keelboat/sailboat that goes on open water, I wouldn't even consider it. Every single bit of these craft gets soaked by salt water, inside and out... Not all day, every day, but at some point. Everything in them also gets knocked/banged up a bit: you come down the companionway, lurch with a wave, put out your hand holding the binos/knife/tool to protect yourself - and bingo, you've just punched a dent in a surface. As you then brush past it in your soaking waterproofs, you coat it in salt water...
If it's one of them, he needs a solid oak table covered in yacht varnish...
If, however, it's a houseboat on a canal, or a big powerboat that putters from a to b in calm weather, you might get away with it, provided all edges are well lipped, and the whole thing is treated with something like a 2 pack lacquer, to make it bombproof... A better option, if you won't use solid, would be veneered blockboard: at least the insides of that can be 'dried out' without having it just go soggy on you...
but at a price 
