Bro THOA, u have hit the nail on the head. Seawater contains a lot of other compounds which aren't so stable, hence they give up their oxide ions to the more reactive iron speeding up rusting. Sec 4 chem stuff lolz.
Fastfret didn't work for me, but Dunlop 65 did. It does tend to 'dry out' ur fretboard a little however, so use sparingly. I think best to go all the way, if ur concerned abt stringlife alone. Keep in case, use Thirsty Hippo and silica gel to absorb water, wipe down strings b4 and after playing, apply string conditioner ( i refuse to call them string cleaner, they dun clean the strings if u know wat i mean ) and u shld get a set of strings that last for 1-2 years? My current set has a 2 mth old coat of Dunlop 65 on it and it hasn't rusted yet. The set is abt 1 year old. I applied it after playing, abt 3 times a wk, b4 'O's. Its a dull colour now, but still smooth and still plays well. It depends on wat strings u use too, i use DR Tite-Fits.