Costa Rica could be a good option! The resort that my husband and I stayed at for our honeymoon was on the beach, and they had a spa on site. We ended up getting coupons for a good discount at their spa for hydrotherapy and a massage, and it was totally worth it. Their beach is good too! Other places to check out would be Dominican Republic, Jamaica, and Bahamas.
I see your profile mentions a Florida honeymoon. If you're considering Tampa Bay, there are some lovely hotels with spas on Saint Pete beach, like the Tradewinds or Don Cesar. The Sandpearl on Clearwater Beach is very nice but that's the most popular beach in Tampa Bay and constantly crowded. Good luck!
Just Said Yes
Mezzatorre Hotel in Ischia, Italy! The best spa around.