The easiest spot to find some productive snorkelling is off the main beach, north of the boat landing. Enter the water approximately in front of the Almarik hotel. If you start north of the beach (about where you can see seaweeds growing at low tide roughly level with Gili Meno’s northern extent), and go with the current back towards to the most crowded sunbathing area, you will likely need to kick only when you want to stop to look at something. The healthy corals are around the area where the wall drops off and the deeper water begins. Nearer the shore you will find only dead coral. Turtles can be seen often, and also the occasional trigger fish amongst the more common reef inhabitants. At low tide it is difficult to get in without reef shoes. Always watch out for potential waves that can push you into the coral that are just below water level. A better coral reef is off the northwest coast, but you have to be very keen to go through the hardship of getting out there. The only access to the reef involves walking over a substantial area of dead, sharp coral, and back again when you have finished. If you are a keen snorkeller the effort is worth it, and you are very likely to have the waters to yourself. Small, purple jellyfish are sometimes plentiful around the island, and they seem to love stinging snorkellers. The reaction is one of considerable irritation, but the stings cause no serious harm. Alternatively, you can take a snorkeling boat tour. Most trips take 4-5 hours with a lunch stop in Gili Air. The costs start at IDR 100,000 and come with mask, snorkel, vest, and fins.
There are several good spots for snorkelling, and gear can be hired at almost any cafe or hotel for about Rp 30,000. The beach in front of Good Heart Cafe on the north western shore is as good as any, and you have a reasonable chance of seeing turtles in shallow water here. Also on the west coast, try the area close to the disused, derelict jetty (formerly the landing point for the equally derelict Bounty Resort). The former pontoon here was sunk and has become an important dive site with a healthy reef life. If you snorkel off the east coast, be careful of boat traffic.
Any of the dive shops will also be able to organise snorkelling trips for you, including hire of gear. Alternatively, there are some good spots where you can snorkel from the beach. Try almost anywhere along the east coast where the fringing reef is fairly healthy and close offshore. Be careful walking in the shallow waters though as there are a lot of very spiny sea urchins in amongst the sea-grass. Equipment can be hired from almost any cafe or shack close to the beach for about Rp 30,000, or your hotel will be able to help you. Please note, the coral reef is a sensitive ecosystem, when snorkeling please swim rather than walk over the shallow reef; will be surprised how much is growing under you, as a rule of thumb, as soon as the water is up to your knees you are safe to swim.