Of course, Abigail herself says "I'm only REALLY smart when I write."
You think of that as modesty, and maybe it is, but maybe not entirely.

My main problem with that scene is the way it's filmed. You get the feeling she could have dropped the keys in the water just before Columbo got close enough. But maybe that's a technicality.

Abigail's intent was certainly to throw the keys in the water but she was then interrupted by Columbo who basically put such a high importance on those keys she handed them over. To us it could be argued to be stupid but to her it was a calculated risk. She hoped that by volunteering the keys she would assuage his suspicion (why would the murderer give him the keys?) because as long as those keys were missing she knew she had no chance of getting rid of him.