Thanks to all for the feedback.
I let the feeder dry out, but this didn't seem to help - the LCD was
blank and the unit still rotated continuously.
So I pulled the whole thing apart and sprayed everything inside with
electrical contact cleaner, and left it overnight to dry.
Tested this morning, and everything appears to be working fine. I'll
reassemble it tonight and see if it's fully functional.
Now I just have to work on a more reliable mount for it, not sure how
many times I can get away with dropping it in the tank :)