Sumatra has several different dive destinations which don't get as much attention as other Indonesian dive sites. That's mainly because they are not as good, however they still have plenty to offer in their own ways and Sumatra has a lot to offer other than just diving, including jungle treks and volcanic lake retreats.
What's more there is still a lot of underwater Sumatra to be explored. For example, Padang, on the west coast, has some decent Indian Ocean dive sites with plenty of fish life and the Mentawi Islands, already discovered by surfers, holds promise for divers prepared to backward roll into unexplored waters.
One advantage that Sumatra has is it's proximity to Singapore and Kuala Lumpur, although transport logistics tend to slow down once on Sumatra. Padang can be reached by flight from Singapore and Air Asia fly from KL to Banda Aceh (for Pulau Weh, however tourist visa's must be arranged at an embassy in advance).
Bintan in the Riau archipelago can be reached by ferry from Singapore making it a popular weekend dive getaway. The diving is decent, if not as world class as other Indonesian destinations. The Riau archipelago, actually consists of around 3,000 islands, Bintan is the largest and the one with the dive resorts. There are many dive sites, most of them fringing reefs. The visibility isn't that good due to the proximity to the Malacca Straits, one of the world's busiest shipping channels. Visibility can be 20m but is often just 5m. However coral is healthy and there are plenty of reef fish and macro critters. It's a great location for training dives. Also, there are seveal ship wrecks in the area to be explored.
The best diving in Sumatra is Pulau Weh, a small island off the north west coast in Aceh province. Aceh province has been politically unstable for years and the December 2004 Asian tsunami also did devastating damage to the area. However the majority of dive sites escaped lightly with most of the damage being done from 0-10m deep with deeper areas of dive sites spared.
Pulau Weh is located off the north western tip of Sumatra and is actually Indonesia's furthest beach to the north west. It's in the Indian Ocean and the diving season is the same as west coast Thailand and Malaysia with the high season running from November to April. There is accommodation and dive centres on Iboih where the jungle meets the beach or on Gapang Beach which has a nice house reef.
The marine life around Pulau Weh is superb on fringing reefs, walls and wrecks. There are over 20 dive sites around Pulau Weh, Pulau Rubiah and other surrounding islands. Many of the dive sites are suitable for all levels of diver but there are also some strong drift dives and some deep decompression dives that are only suitable for experienced divers.
Batee Meuduro pinnacle is located about 1 hour south of Pulau Weh. The isolated pinnacle attracts large schools of pelagics as well as sharks. It is a favourite dive site in the area and has the best visibility.
Pantee Aneuk Seuke (The Canyon) is a good dive spot to see manta rays. Napoleon wrasse can also be seen. Pantee Peunateung is a deep drop off to 70m where big schools of pelagics can be seen including barracuda and trevally. Currents can be strong here.
Batee Gla is a also a good drift dive with strong currents. Rock formations create swim-throughs and archways framed by gorgonian seafans and large schools of fish including snapper, fusilier, bannerfish and surgeonfish.
Pulau Seulako offers some good drift diving with the chance to see big fish. Arus Balee is a drift dive where the nickname given to the current hints at the strength of the flow. It is called the Acehnese Arus Balee, or Bastard Current! A rocky pinnacle in the passage between Pulau Seulako and Pulau Rubiah atracts bigger fish like barracuda, trevally and sharks.
The Sophie Rickmers Wreck sits in 55m of water in Pria Laot bay. The shallowest part is the wheel house at 37m so this dive is not for beginners. She was a German cargo steam ship built in 1920 and sunk by her own crew on 1 May 1940 to prevent confiscation by the British navy. Also in Pria Loat bay are underwater hot springs where sulphur steam produces bubbles.