There was a question on the Physics StackExchange, how to calculate the resonance frequency of a string under water. The only information given was that the frequency in air was 280 Hz. The solution was 240 Hz.
This seemed a bit strange, so I decided to try the experiment with our sonometer. I first tuned it to 280 Hz in air. Then I put the whole setup in a trough with water, see image below.
It this point I started to to realize what was going on. And indeed, the resonance frequency was lowered quite a bit, in this case to 250 Hz. The oscillation also became much more strongly damped.