Pushing the button while submerged shouldn't damage the device. The whole purpose of this unit is to be able to record laps while swimming. It's also *supposed* to be waterproof.
Did you ever figure out how to fix it? Mine has started doing the exact same thing after taking a shower.
I just checked the manual for a swimming-capable Polar watch (the vantage m). Same thing. Don't press buttons under water. Also, I have a 50 m rated casio and a 100 meter rated timex watch. Again, manuals…
That's cool, but if you look into the manuals for those two watches you will find the same precaution.
The problem exists, because designing waterproof buttons that are nice and easy to press is a challenge from an engineering point of view. Look how different the buttons on the 310/910 are from buttons on more recent garmin watches.