Which side are you talking about?
On the North-East of Sri Lanka, there is a mix of reef, slope and pinnacle diving with the usual tropical fish hanging around. I believe it is possible to see some pelagic when you got lucky but I didn't see any when I dove there (2 different short trips in this part of the country)
On the south coast, you have more options in term of diving, with even some small wrecks which are now completely destroyed by the current and time.
I remember doing some very nice drift dives with decent corals and fish schools. And as said previously, I got lucky enough to see Manta rays on 3 different dives, and 2 consecutive days.
I dove twice in Sri Lanka, in October 2013 (Trincomalee and Unawatuna) and in August 2016 (only Trincomalee)