The stainless mini is fairly rust-resistant, but my stainless Ranch Rifle did get some light surface rust simply from being in the trunk of a car for a month in humid northwest Florida; it wouldn't stand up well to salt spray. If I were taking a mini on a boat, I would definitely go with a Choate pistol-grip synthetic stock (won't soak up salt water, and gives you a more secure grip); I have the Choate "E3" style stock on mine. After September 14, you may be able to get a Butler Creek synthetic-and-stainless folder.
HOWEVER, as an owner of both a stainless mini-14 and a SAR-1 (Romanian AK clone), in the situation you describe I would probably ship the SAR and a couple of mags off to Robar, have it coated with the dark-colored NP3 coating (rather than the silver), put a synthetic stock set on it, and go with it. Not only is the AK probably more abuse-tolerant than the mini, and less prone to get gunk in the action because the safety covers the charging-handle slot, but a major weak point of the mini-14 system from a marine-defense standpoint is the magazines. Mini-14 magazines, and AR magazine for that matter, are simply delicate. If you get a chance, compare a mini-14 or AR magazine side by side with an AK magazine; the feed lips of the former are very-light-gauge steel, while the AK mag's feed lips are more than 1/16" thick and nigh indestructable. If your magazines get banged up by rough handling, being dropped, etc. and won't work, you're sunk (no pun intended). Just make sure your mags and mag springs are corrosion-protected (even if you use synthetic AK mags, the springs could rust).
Also, the AK's tapered rounds will probably feed better when dirty than the .223; the AK makes .30-caliber holes in a boat instead of .22 caliber holes; and AK magazines are somewhat quicker to change than a mini's (though not as fast as an AR). Unless a politically-correct appearance is important (in which case a mini-14 with a synthetic Monte Carlo stock would fit the bill), I think a corrosion-protected AK would be the better choice for me.
Also, don't forget one thing that may be important in a crisis, perhaps even more important than which rifle he chooses--training. I don't know how "into guns" your stepdad is, but if I were him I'd try to get in a Tactical Carbine course from John Farnam (or whoever is most convenient). If he is ever attacked, he'll have the advantage of playing defense, which is a little simpler, but the bad guys in this situation may have the advantage of experience; I doubt he would be the first armed boat owner they had ever encountered. Just a thought.