Beauty is right. Maldives are excellent. A little more expensive.
But Sri Lanka of course has an entire country and culture to explore at the same time (Not that Maldives dosn't!! Just most people go there specifically for the diving/surfing and beaches).
The best diving in march will probably still be on the west coast, but the east should be opening up by then, ie Trinco area.
South of Colombo is the place to head to. But there are operators out of the capital also.
Bear in mind "side trips" are not really an option anymore as the Indian govt has changed the visa rules so once you leave the country, you must stay gone for 2 months, even if the entirety is within your visa period. Better to do your trip either side of your India time.