I'd use whichever one was the easiest to remove once repairs are started. Fingernail polish will eventually wear off on its own, but removing it would require nail polish remover, some of which contain acetone ... which is not recommended for gel coat or fiberglass in a concentrated form. I'm not sure about Flex Tape, but they do have "removers" for adhesives.
If the gash is not thru the fiberglass ... personally, I'd opt to leave it as is. Repairs are going to warrant cleaning the area anyway, so any "waterline scum" would likely be easier to remove than any sealing product.