Oceanic Scuba Dive Center
This is the only dive operation in the country, and is run by a friendly group of ex-navy divers. At the moment they only visit one site at the southeast of the island off Chera Dwip, which is unspectacular but still makes for a nice boat ride and the most activity you could hope for during your stay on the island. They're working towards buying a 'German' boat which will allow them to explore sites further out to sea, which may yield some better diving and more sea life. Prices are a little high and negotiable, but consider that you're contributing to these guys' livelihood and towards expanding the opportunity for future diving. They've also got one bamboo guest hut with two beds if you're diving with them, and they'll cook all your meals for you — an alternative to the lackluster budget accommodation on the island. Food and lodging are by donation. On a safety note, this is nothing like a PADI certified dive center, and beginners should take care, especially alone — a guide/buddy is not included though there's a good chance one of the guys will still come down with you.
Walk the beach
To the southern island of chera dwip, accessible on foot along a narrow strip of land at low tide, or via a boat shuttle tk 5/person at high tide. beware of sharp rocks if wading through the water; sandals are ideal. walking the length of the island takes around 1.5 hours. take some sunscreen and water though there's an occasional shack selling drinks, coconuts and snacks.