That's the trouble with many large discount marine stores. The staff doesn't know what the hell they are talking about. Thank God this is not true in all places or with all staff.
The bozo who recommended 3M 5200 should be keel hauled. That is an adhesive and as someone else pointed out, if you ever want to remove the fitting, forget it. It should only be used for permanent applications. If you like 3M products then use their 101 polysulfide caulk. It is designed for under the water line use. If you go with Boat Life, then use their Life Calk (spelled that way). Polysulfide is a Thiokol product that was first introduced as a two part product. But, for many years you can get it as one part in tubes or cartridges. It is very sticky stuff to work with, but is recommended for under the waterline. It was used on wooden carvel planked boats along with caulking cotton in the old days. It is often used on teak decks and comes in a variety of colors. But, for through hull fittings it is the best choice. It actually can be used to install a thru hull and then immediately launch the boat. It cures quicker under water (with the absence of oxygen). On the inside of the boat, where your nut screws down, you can even sprinkle a little water on the caulk and that will help it cure quicker on the surface. It is a slow cure out of the water and eventually can be sanded (as when used on teak decks). And, as someone else suggested, silicone should not be used under the waterline either. The boat life people have a great Life Calk solvent that cleans up your mess quite nicely in comparison to acetone a rag or the other goofy stuff people try to use. Power boaters with aluminum fishing boats will use silicone under the waterline, but what do they know? We're sailors!!!
