The GFI can be tripped by dew or condensation, so it's very hard to get something that has been fully wet dried out enough to work on a GFI circuit. Leaving it out on the deck in the sun might help.

When I installed my winter cover, I left the extension cord on the pool deck (unplugged) overnight. The next day, I went to start drilling and nothing. Dried the cord with a towel and still nothing. Finally got a hair drier and dried both ends.