i am pretty sure this will not be of much help, bit it may be better than nothing.
I was in Koh Samui in July 2010 and did a few days diving. Unfortunately I cannot find any record of the dive operator I used. Which is a pity as I was reasonably happy with the service. A google search doesn't bring up any names that look familiar.
A few things I did notice that might help you sort out a good operator though or at least set your expectations.
All the diving I did was at Koh Tao or Sail Rock, which was about a 60 km boat ride from the Northern side of Koh Samui. The boat only had one speed, flat out.
From memory it was an hour and a bit in each direction.
The boat did not appear to be linked to one operator, but seemed to be used by a number, each with their own dive guide. The boat supplied a common lunch, which was quite good
Most of the guides seemed to be expat Europeans, at least on our boat.
Koh Tao dive sites were crowded, lots of boats in the same spot, putting down lots of divers. It may have been the time of year, or just a bad week, but I did not find the diving itself all that great, I may have been prejudiced by being in Chuuk in November 2009 though.
When I went I just googled Koh Samui and dive charter, then emailed the operators and made a choice based on the reply. I know the operator I went with did not even want a deposit.
I guess there is always Trip Advisor, if you do not get any one with recent experience replying
Hope you have a great trip