It's just like throwing a power pole into the ocean.
Tamagotchis are electronic devices. Nothing, except an extremely thick and bulky waterproof cover, could waterproof it. Electricity and water don't mix.
^ No, they are not. If you get a little water in it, it should be fine, but a lot and it WILL break. Like if you take them out in the rain, they'll be okay. Don't go throwing them in the ocean though.