[benz]

In my experience, and maybe I'm just bad at this, 3M 5200 is the only stuff that could keep water out of my boat. I did a good bit of business in Florida rebedding deck hardware whose bedding had leaked, in some cases catastrophically damaging boat interiors. Even when I pulled off hardware that had been bedded previously with 5200 (even that can leak if done poorly, of course), I never brought up chunks of gelcoat--I'm sure it happens from time to time, but 5200 is NOT guaranteed to ruin your gelcoat. I always rebedded with 5200, and none of my clients ever came back to complain of leaks.
I will say that if you're going to do it, do it right--slather the stuff everywhere, make sure it squishes out from under the fittings when you tighten them down, and then just wipe up the excess with paper towels and mineral spirits (note--do NOT use acetone! it will work, but not as well, nor as economically). If you bed your fittings properly, you may never have to unfasten them again, and gelcoat damage will not be a concern at all.
Happy gooping!
Ben