So yesterday MrsWD spent a good 20 minutes happily splashing around in the freezing cold water and declared she couldn't find the damn screw. Today I reluctantly put on the mask and snorkel and found it in about 5 minutes. I think she was having too much fun and I was motivated to get the job done and get the hell out.
It's stupid to use these screws for the purpose. I'm replacing them all with 3/16 pop rivets and sleeping well from here on out.