We were there in Arpil. Don't take the Princess tour, it is silly to pay more, it is 7 dollars for a taxi( 14) then 3 bucks to get in. So for 17 you can go in the morning when it is way less crowded. That's waht we did, we stayed 3 hours or so , then left when the tours showed up and crowded the place up.Be sure to walk along the beach to the far side where the cactus grows.
Next time will try Coki though, there was no snorkling at Magens.