There are really very, very few colleges which are truly on the beach. The ones which are in coastal areas usually require transportation and time to get to the beach.
The only exceptions which are truly on the water that I can think of are New College of Florida, Roger Williams in Rhode Island which is on the bay, and Hobart & William Smith which is on Seneca Lake in NY. Fairfield University is not on the water but it does have some housing on the beach which connects to campus via a shuttle. I’m sure I’ve missed some place, but they really are rare.
I have a GD who wants the same thing, so I sympathize, but I have to say that it really is a silly idea. Any kid who is focused on that is focused on the wrong thing IMO. Summer jobs are plentiful on Cape Cod and other beach communities. Some even have employee housing. Do the beach thing in the summer.