I'll second the notion for the Westin, had many superb stays there over the past several years. The Hyatt is equally as nice, but as others have said the regualar rooms are across the street from the beach. Also, the Hyatt is a larger hotel and has less actual beach space than the Westin, so the beach area can get very congested at times.
I like going to the Caribbean to sit on the beach and take the occasional afternoon dive trip, and for that Grand Cayman is the number one place.