Sorry to hear. Sounds like it is fatally faulty. Water? After the usual soft and hard reset, contact Garmin. If you are still under warranty, you should be set. Otherwise (at least in the US) they will likely offer you a refurbished unit for a reasonable price.
It's meant to be water-resistant, but it also says in the manual you are not meant to press the buttons underwater. Some people have seen issues with this ie water ingress
CW