If, like me, you have young kids on the beach and you have to divvy up time out in the waves with policing junior's attempts at trashing everyone else's sandcastles, then you need to know when to head in-shore and lend a helping hand. Or in my case, let my lovely sea-nymph of a wife get out there with her body-board.
I once lost a supposedly wave-proof watch in a wipeout on a beach in Donegal. So many hundred pounds of Swiss craftmanship turned into crab bait in an instant. Lesson learnt though,so these days, I buy these cheap and cheerful "Surf Watches", strap it on over the old wetsuit and go get wet.
The US company that markets them is pointed in your direction here...
One tip, smear a bit of wax over the timepiece for some extra shelf-life.