The Colombo seaboard down to Panadura has the highest number of known shipwrecks (30) and best dive sites. They are mostly latter-day shipwrecks with about 25% from World War I and II. Shipwrecks with treasures, priceless ceramics and coins from various maritime nations lie off the ancient ports such as Galle, Mantai and Trincomalee. The most studied of these ports, Galle is known to have as many as 26 maritime archaeological sites.
On the eastern seaboard at Trincomalee, Batticaloa, Vakarai and Kalmunai are where most of the World War II wrecks are found. The two Basses reefs in the south are known to have the most treacherous seas during the two monsoon seasons. They are home to priceless archaeological wrecks dating prior to when the lighthouses were built in the 1870s.
This is one of the most comprehensive sites I have found that explains it all better than me includes maps and videos. Follow the link Shipwrecks.