I lucked out, the guy who installed my HVAC said not to put a return in my fishroom for that very reason.Ill tell you from experience in going through 2 AC units in 4 years both Mitsubishi split systems in my sump room. NO i repeat NO AC system in that room will survive the saltwater humidity even with exhaust fans churning out the indoor air. I had to design a AC system that is a central AC unit, outdoor condenser, indoor regulator. The difference is that my return to the unit is not in the sump room. All AC guys were telling me you cant do that. I told them you know your business and i know mine. Fresh air from another clean air room takes in the air and blows the cold air in to the sump room. No more salt creep humidity destroying my units anymore. Im going on 7 years now with this unit. No rust or problems anymore.
I can add a lot here nut i see most people brought up the main issues.
Just make sure your sump room is super large, youll be spending more time there that you will anywhere else in your house.
I wish I had a bigger fish room. I filled it up with a big tank. Not much elbow room but I can manage.