D.C.
I just came back from the Marriot on Key Largo. It was fabulous. We loved it. There is a dive shop on the property for snokel trips. We spent a lot of time by the pool and on the (small but nice) beach. I did not try out the spa, although they do have one. The bartender at the pool makes a good mudslide.
We went with our kids, but there were a lot of couples there without kids as well. We had breakfast at the resort restaurant, and the buffet was good. We didn't have dinner there though so I can't comment on that - our kids are young, and we went to more informal places for dinner.
I also like Key West, but that is more about nightlife than relaxing. No good beaches on Key West. Of course there are snokel trips that you can find though. I don't have a specific resort recommendation for Key West - we always stayed in hotels that were just hotels, and spent little time there. Spend most of our time either on a dive boat, or on the main street having adult beverages. Key West is NOT a place to go with kids, IMO.