There was another cover/strainer over that sea strainer...(blue rectangle)...that I took off...
I think that bubble is part of the thru hull, But I can take another look...
Using Evercoat 574 Marine Fiberglass filler supposed to be for below water use...Don't know if that's different than 3M product.
As far as
epoxy vs poly...sounds like there is no expert schedual for permenent
repair of blisters so whatever I put on is going to fall off...or not...