covered? Sounds like blisters
. Not a good thing for either a water
tank or hull
. If your boat is a pre 1990's or their about... it may be the osmosis
I've had a small sailboat that had a minor problem on the outer hull
and it was a job to cut/ grind, drain the "sores" and allow them to totally dry out then grind them flush and coat with something like a West Sytems.... but that is for bottoms.... Not water
If what you have is the osmosis
problem where the fiberglass
is breaking down...(BTW it probably also smells bad) this would not be good for a water system that you use for drinking water
. You may have to look at replacing or some how isolate the walls from the stuff and the water.
Hope someone comes up with a better idea but sounds like blisters
to me. Good luck!