Have done it many times with my exwife, we really enjoyed it in the water even on the shore while putting to her back some suntan lotion making her ass orgasm as well as me giving her a good creampie. She loved to be fucked in public.
I've fucked in the water at a hotel beach with more than a dozen people within sight, 100' or less away. However we kept all the action underwater and did it with her bouncing in my lap. It was pretty hot although it was pretty hard to cum. My recommendation for fucking in the water - either prelubs or take lube you and make sure you use silicone based lube also sold as anal lube. It'll make it a lot more comfortable to penetrate and keep fucking to climax and hey it's silicone based so you can have some anal play too.