A cozy hotel on the beach
Corn Island, just off the Atlantic coast of Nicaragua (and a short flight from Managua with La Costeña airline), is one of the few Caribbean paradises that is still largely "undiscovered" (though I'd bet that's going to change soon).
One of the best beaches with the clearest, calmest water on the island is in front of the Picnic Center, a hotel and restaurant. At a very reasonable price, you can rent a cabaña directly on the beach - mine was a very small single/double, but wholly charming and comfortable, with attached bathroom. At night, I could hear the waves as I lay in bed - a lovely way to fall asleep. You can also enjoy the thatched-roof restaurant, which is open for breakfast, lunch, and dinner. They have small thatch-roofed gazebos nearby where you can eat and drink, in case you'd rather have a more intimate table with just your partner or the friends you're traveling with.
In the morning, just change from your pajamas into your swimsuit, and wander out for an early swim - you'll only have to take a few steps from your cabaña to get to the water's edge.