5200 for below the waterline, or other permanent mounting. Silicone for sealing or sticking things you will be moving. I once had a boat that had seat pedestals screwed down and bedded in silicone. I removed the mounting screws and the silicone would have held by its self. I had to hammer a putty knife under it to cut the stuff loose.
I used 5200 on some cad plated lag bolts holding the bottom bracket of a swim platform, I covered the bolt heads so they would'nt rust. No rust on it when I sold the boat 14 years later.
Everything screwed below the waterline: transducer, trim tabs. Glued down washdown pump in lieu of screws in the rubber feet. Fixed stripped out screws. Works good on wire connections. Stuck the little bracket hanger button back on my radio mic.
Everything below the water line and in bilge areas and never on something that I don't plan on feeding and making a pet out of, because it will be there forever! For things you may remove or service they make a 3600 that is a lot less permanent but still offers good protection.