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The most popular wrecks are the Pecheur Breton, Medhufaru and the Chief Dragon but there are several others as well. Many wrecks are fully or partially intact, offer penetration and are located in the 20-35m depth range. Currently there are 2 dive centers (Island Scuba and The Dive Shop Sri Lanka) operating in Colombo. Both are based in the southern beach suburb of Mount Lavinia. A third, Colombo Divers is currently closed and I am not sure if they will reopen. For full disclosure I am a partner in Island Scuba which we started last December.
Generally early December has good conditions with calm mornings and afternoon winds. Visibility in the 10-30m range depending on site. I'm more than happy to answer any questions and if you need specific information about Island Scuba just PM me.
Cheers,
Nishan
The ones I know are International Dive Center and Poseidon Diving Station. I have not dived with their Trinco shops but know they run well organized and safe operations down in Hikkaduwa on the south coast. Unawatuna Diving Center runs a good op in Unawatuna and has a branch called the Nilaveli Diving Center. Sri Lanka Diving Tours also seems to get good reviews. The owner tends to look for new sites and is a bit more adventurous than most others.
Trinco diving is mainly on shallow hard coral gardens and rocky reefs in the 10-20m depth range. You can snorkel at Pigeon Island where you can see lots of blacktip reef sharks. Go early to avoid crowds and get better shark sightings. Marine Park fees apply. General fish life is low on most sites but you can find lots of morays, anemonefish, nudibranchs, cleaner shrimps, anemone crabs, pipefish and even a harlequin shrimp. Swami Rock has interesting topography and a rock wall going down from the surface to around 20m. Another site I like is Kotuwe Gala (meaning Fort Rock). Its located close to Swami Rock and the Dutch Fort in Trincomalee town. There is an American Revolutionary War era British war ship off Uppuveli but none of the operators seem to dive it.
Most shops are in Nilaveli and Uppuveli, north of Trincomalee town. Dive shops located in Uppuveli dive more towards Trincomalee town and Swami Rock. Those in Nilaveli tend to dive more around Pigeon Island.
August can be a bit windy with low viz but hopefully you get decent conditions.