I would not worry so much about the rain, as I would worry about the rouge wave. Some machines are built for use on land, while other machines are built for underwater use. The Sovereign is a land use machine, and I would not risk lousing the machine to a rouge wave, or a slip and fall in the shallows.
If you are serious about hunting in the water, then my recommendation is to purchase a machine that is designed for water hunting. I use the Minelab Excalibur 1000 primarily for underwater hunting. However, I also use it to hunt the dry beach from time to time. She is not as light as most of your land machines when she is out of the water, but she works just as well at finding treasure on the beach, as under the waves.
Q