Yeah there is some mediocre diving as well as some world class diving. Sri Lanka is not very developed in terms of a dive industry so information is poor and many operators tend to cater to walk in holiday divers rather than serious divers. March is actually a good time to dive the west and southwest coast. The sea is also good for diving on the east as well as the Basses Reefs on the southeast so its one of the best months for diving all around the country. For the latter two areas however, there are logistical issues. East coast diving is run by the same west/south coast operators who shift seasonally so most are based on the west side in March as its season there. For the Basses, trips are run by a few south coast dive shops but mainly around early April to coincide with local holidays as the trips mainly cater to local divers.
The east has the better hard coral but some of the best areas are off the beaten path and not always accessed by dive shops. There are some stunning coral gardens in 2-5m depth that are great for snorkeling. Pigeon Island off Trincomalee is easy to access and has decent coral growth and lots of reef sharks but expect to pay some very high Park fees. Good thing is you can just snorkel and see a lot and save on dive costs.
Now for the diving. The south coast diving in popular areas like Bentota, Hikkaduwa and Unawatuna is mostly around rocky reefs and pinnacles. The best sites have decent marine life but coral growth is not great. There are some interesting wrecks with historical significance but they are broken up and dont offer much penetration. Colombo has some excellent wreck diving, many intact with good penetration and also decent marine life. If you like wrecks this is the best area on the west coast. The reefs are generally flat sandstone ledges with soft coral and decent fish life. Negombo has reefs similar to Colombo but I think with better fish life and some interesting macro. People have spotted seahorses and sea moths and other cool stuff.
These are the dive shops I can recommend. These are shops that I have dived with and does not imply that others are bad. There are a couple I would recommend against but please PM me if you want that info.
Bentota - Scuba Lanka.
Hikkaduwa - International Diving School and Poseidon Diving Station.
Unawatuna - Submarine Diving School and Unawatuna Dive Center.
Colombo - Colombo Divers.
Negombo - Sri Lanka Diving Tours.
The best diving in Sri Lanka is focused on wrecks. Everything from wooden sailing ships to WWI, WWII and modern ships. Some are world class dives, and all the wrecks have a story around them and have not been sunk intentionally. The best and most famous is the HMS Hermes, which if you are tec certified is a must do. There is some good reef diving but doesnt compare to better known destinations in Asia. The best reef diving is either very shallow (<10m) on the east coast or deep (>25m) on the west coast.
Just bear in mind that the industry is not well developed in SL so many shops can be fairly basic and the guides are not always great, especially for finding macro critters. But they are generally safe, the dive sites are not crowded, and in most places you get a personal service. Boats are not always ideal but this is also due to various restrictions brought in during the war. And in places like Negombo and Colombo they work with small boats that have to be beached and launched through surf. If you go with an open mind you will have some great dives!