If you intend to stay for more than 2 days bring your own snorkeling equipment. There are masks and fins in the hotels but they are not in a good shape. The crystal clear water and the colourful fish gives you the impression you are swimming in the scenery of 'Nemo' film... The best island near Porvenir is El Perro which is surrounded by warm turquoise water with a shipwreck full of beautiful fish very near the island.
You can also arrange with your boatman to take you out fishing. In that case bring a hat, put tons of sunscreen and a wrap cloth to cover your whole body. The sun is ruthless!!!