Many of the best diving sites are around Ko Tao, the smallest of the three large inhabited islands here. My dive boat was fairly large (larger than the one in the photo), with a lower "wet" deck for the equipment and an upper "dry" deck for rest, meals, and sleeping.
Diving is mostly corals, but they have also sunk various concrete structures and seeded them with corals to create artificial reefs. There are hollow concrete boxes and suspended hoops for practicing buoyancy skills, but also some steel wire structures resembling boats and houses, concrete sculptures, even stair masters and weight racks, and a family of toilet bowls with their seats gently flapping in the current.
At a depth of 30m lies an old warship, also deliberately sunk there to create a reef. Many fish have already made it their home. Divers pose for pictures at the cannon, and there's an actual line of divers waiting to swim through the bridge of the ship. Unfortunately the no-decompression time at that depth is quite short - the deeper you go, the more air you use.
I'll post some underwater pictures later.