A few kilometers out of Playa Girón, near Bay of Pigs in Cuba, there is a place called Cueva de los Peces. Here you'll find a brackish water cenote and a small SCUBA operation.
The cenote is reason enough all by itself to stop of on your way to wherever. It is set back far enough off the road for the tropical forest to absorb the sounds of traffic and as you walk in the scenery turns peacefully idyllic. The cenote is the size of a small pond but its depth is unknown - far deeper than a snorkel would allow and the divers who have dared have turned back before reaching the outlet.
One can bathe on the banks or dive in and, if you come during rainy season you're in for a treat! When it has been raining the brackish water that comes in from the ocean through the bottom of the cenote supports a few inches, maybe a foot, of fresh water on top of it. And, it may as well be oil and water for the difference in perspective and the different creatures that live at the two nearby depths!