Koh Yung offers some of the best snorkeling to be had at Phi Phi, particularly in terms of coral. There aren't any beaches to talk of on this little island, it's very rocky.
The way that I do it is by swimming from a longtail just off the shore.
It's one of my 'must visit' sights when we visit Phi Phi.
Your local longtail driver will be able to take you to the reef which is covered with hard and soft corals. In addition to schools of barracudas, moray eels and lionfish, you can also watch leopard sharks sleeping on the sandy bottom.
Just 1km north-west of Mosquito Island lies Mosquito Pinnacle. This site drops down to a depth of 25 metres, so it's more attractive to divers, but it's still worth visiting to snorkel. Here you can find multi-coloured soft corals, sweetlips, giant morays and blue spotted stingrays.
Though it's very close to Koh Mai Phai, these two islands are very different and offer contrasting experiences.
But for me, Koh Yung, (Mosquito Island), is an absolute must for any keen snorkeler, not least because it's so much less busy than Koh Mai Phai, (Bamboo Island), which sometimes is jam packed with day trippers.