Yellow /Brown Stain above Waterline

[abl1111]

What is that yellow/brown stain that I get every year on the gelcoat, right above the antifouling paint ? Every season it appears.

My water line could be raised for sure, but the water line is always above the brine, albeit only 1" or so.

My theory is that the 42 year old gelcoat is porous - even though I wax it well, it allows for the stain. It is easily removed with muriatic acid but I'd like to prevent it in the first place.

Ideas ?