I dont know about the 250, but most detectors are waterproof at the coil. The only part that is not is your display. My Bounty Hunter is the same way, and it works fairly well in salt water, but doesnt ground balance as well as my ATPRO, so it gives alot of noise. When you hear a good target, you will know.
I would suggest staying in dry sand until you get used to the detector....bury a couple different items (coins, tabs, gold) and see where it registers in the sand. Then as you grow better, start going to the water. Being able to get your find in the water is the tricky part, just focus on not dropping your 250.
HH and keep us posted on your finds!