Why it's protected
Los Arcos was declared a refuge zone for the protection of marine flora and fauna in 1975. That status is the reason it still has fish: no commercial fishing, no anchoring on the reef, and rules about how boats approach.
For you it means two things. The snorkeling is better than anywhere else nearby, and there are limits — you'll be asked not to touch or stand on anything, and not to feed the fish.
What you'll actually see
Schools of sergeant majors and surgeonfish right under the boat, pufferfish tucked into the rocks, moray eels if you look into the cracks, and rays cruising the sand. The granite walls drop steeply, so you get the odd sensation of floating over a cliff.
Above the water there are pelicans and blue-footed boobies on the rocks, and the arches themselves — some big enough for a small boat to pass under.
Visibility is best in the morning and after a few calm days. After heavy rain it drops, which is worth knowing if you're booking in summer.
If you've never snorkeled
This is a good place to start. You enter from the boat into calm, warm water, there is no current to fight in the sheltered side, and you can hold a float the whole time.
Gear and life jackets are included on our tours, and the crew gets in with first-timers. Kids from age 6 do it regularly. If you'd rather stay on board, the arches and the birds are worth the stop by themselves.
The night version
The same spot after dark is a completely different experience. Los Arcos is far enough from the city lights that on a dark night the bioluminescent plankton lights up around every kick and hand stroke. We run that as a separate sunset trip.
