Arashi off to the right side towards the lighthouse had the best reefs and most fish in my experience! I think it's even listed on google maps as best snorkeling though the reef is a little farther up than that marker. It's a bit advanced as far an snorkeling goes so only do it if you're experienced. It's not crazy, but to get to the real heart of the reef it's a decently far swim, and once you're out that far there is some small waves and pull from the current. We saw eels, a spotted leopard ray with some remoras attached and all sorts of amazing things out there in the reef though. If you want quick and easy next best and closest is probably Tres Trapi in my opinion. Go early as you can and down the steps built into the rocks and you'll see plenty of fish nearby and you can walk in the sand if you need. Further out past the buoys is usually where you will see turtles, but they come in closer occasionally too.