in March and April I did a lot of Reefcheck dives at the Similans, to get a Status on the Divesites for a Diveshop in Khao Lak.
All in all I can say that 80% of the Dive Sites at the Similan Islands are still in very good conditions. Only a few spots are partly or completely destroyed. But all in all you still can do a lot of amazing dives in the andaman sea.
A short list of the damaged dive sites:
Deep Six - almost destroyed / none of the Live Aboards go there anymore North Point - just a few brocken corals Snapper alley - almost destroyed East of Eden - 80% not affected - but chinese garden ist alsmost destroyed but you can still do nice dives there Donald Duck Bay - a lot of brocken corals in shallow water area Elefant Head - just a few brocken corals
Similan Island No. 1-3 - still closed by marine park Fantasea Reef - still closed by marine park Deep Six - will be ofically closed soon
All the other dive sites are unaffected by the Tsunami.