I did two dives with Unawatuna diving centre, and the staff were always helpful and friendly. It is a great place to go if you love wreck dives, as there are around 8 in the close area. I enjoyed both of the dives, which are conducted in small groups of no more than five, making it less intimidating for people travelling alone, like me.
However, the standards for safety, I didn't feel were as high as other dive centres I have dived with in Zanzibar and the Maldives. The equipment was all good and well serviced, but the divemaster didn't ask us to inform him when we were running low on air with enough time to complete the dive comfortably, so someone in my group had to ascend while sharing air with the dive master. I know this isn't the end of the world in terms of safety, and the diver could have checked his own air more frequently and made his own decisions, but it's not something I have encountered before and would rather not again!
That being said, the prices are good and it is definitely well worth a visit if you are in the area.