I would recommend any of the Sandals resorts as they are built especially for couples. The beaches of Oaxaca are also beautiful and have beautiful blue water. If you are a surfer, Puerto Escondido is absolutely the place to be and is very inexpensive.

The Yucatan is also home to beautiful beaches but had become pretty touristy. Cancun is home to Spring Breakers around that time and Cozumel would probably be busy then as well. Check out TripAdvisor on individual hotels and resorts.

Buen Viaje!