Yes, pulling out the keys is very stupid, at least to our thinking. So are many actions of the murders - everything Nelson Hayward does is ridiculous for instance. Actually many of the police that Columbo works with are 'stupid' in that they just do not get anything Columbo is talking about.

I think that the 'stupidity' aspects of Columbo are actually planned. The writers are not 'stupid'. They know that the viewers, at least myself, want to be able to compare and contrast themselves with the characters and to feel a little superior to them. So they make characters do things that we would not do, and we can feel that we would have pulled off the murders and actually might have beat Columbo. We can even feel superior to the cops, and think that we would understand him and really help instead of showing ignorance and sometimes contempt for his ideas.

But in the case of pulling out the keys - that is really stupid.

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.