Cocky2 said..
Never push buttons while on water as they are not waterproof.
Many of the issues I have seen are from salt getting into contacts.
Also have had the issue with zero readings in the afternoon however distance is still recording.
That's the big one Cocky, not only when on water, but if the watch is wet when you get back on land, Water can collect inside the button housing, pushing the button lets it into the watch. Mine got water in because I was wearing it over a bunched up spray jacket, when I crashed the spray jacket pushed the buttons underwater.
The zero readings are a poor satellite problem, the watch has a smaller antenna than the other devices, and the underhand grip doesn't help either.