Thats an interesting scenario, I've done most ofmy training flight down in Miami and would always fly up to Fort Pierce on cross country flights . Midway along the route ATC would give us an option to either fly at /above 2500 ft or below 1000 ft . A lot of the guys would descend down to 500 ft just to get the thrill of being so close over the water and of course South Beach and all that jazz. Personally for me I always chose the higher altitude incase the engine just quit , constantly reminding myself of a plan of action if Lady Luck decided to part ways.The answer to this really depends on how populated the beach/water is the basic idea would be to steer clear of as many people possible before making the landing. As an afterthought ditching somewhere in the deep would be another option if I could swim to save my life , else I'd be worried about sand in my eyes.
cheers !