Mahahual sits on the Mesoamerican barrier reef at the point where it starts running south towards Belize, and the diving is the reason to be here. The reef is a few hundred metres out, shallow and in good condition, and on most days there is nobody on it.
The exception is the cruise pier, a couple of kilometres north. When a ship is in, the malecón fills and the nearest dive sites get busy; when one is not, the village is almost silent. Check the schedule before you plan a day.
The serious trip is Banco Chinchorro, two and a half hours offshore: the largest coral atoll in the northern hemisphere, a biosphere reserve, and a graveyard of wrecks on its windward reef. Weather closes it often, so allow spare days.
Inland the Chacchoben and Kohunlich ruins are a short drive, and the lagoons and mangroves behind the village are worth a boat.