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The best area to go snorkeling on Gili T. Store your stuff at one of the tiki huts on the beach and swim out here for a good time. The current here runs North-South so walk up to the North end and flow with the current South. The further away you get from the shore, the nicer the coral gets. You want to be right at the edge where the shelf drops away to the sea. You should be able to rent snorkel gear for 25,000 to 40,000 from any of the local shops. You can also just buy your own gear for not much more. The water here is nice and clear, but be sure to get away from shore. And be sure to rent fins, super important.
On shore, there are a ton of bars with tiki huts and nice seating. Grab yourself a tiki hut if you can to leave your stuff in, have a few drinks and some mushrooms, go snorkeling. Great plan. If you are traveling solo, I highly recommend finding some other people to swim with, just in case. Be smart. Everyone is super friendly so just ask if you can join a group of other people.
Another great option that I highly recommend are the guided snorkel tours. Typically 75,000 a person, it is totally worth it. You do 3 dives and they usually change the locations. You get to dive at all three Gili islands and you have lunch on Gili Air. Its a fun way to meet people and just something you should do. To get one of the tours, just talk to any of the local kiosks, they are everywhere.