Okay, there must be something in the air here in MD because I also started having this problem for the first time this year! I can’t even list all the disgusting things I’ve found, even just a few hours after last checking or cleaning the water. Mine is a Nelson waterer so it’s very easy to clean at least.
I don’t want to jinx myself, but the situation did improve after a few weeks. Initially I felt bad tossing their presents, so I would move them to the top of a fence post farther from the waterer (one of their other favorite storage/display areas) at least the first time. If I found the same disgusting thing in or near the water a second time, I’d throw it into the bushes. I narrated this for the crows, out loud, like a total crazy person. I also thanked them out loud whenever I walked out there to a clean waterer, because crows are smart and they can hold a grudge. Crazy, I know. I hope my neighbors heard.
My friend recommended setting up an alternate shiny bowl or altar for them, but I haven’t done that yet. And knock on wood, it’s happening a lot less now than it was.
Photo of the culprit pecking at a lovely dead baby bird, back in May: