Hopefully this is still relevant. My husband and I did our open water course in Koh Tao back in June. We went through Bans diving school, who were really good with professional instructors and great equipment. Accommodation was included, basic but you're out diving all day, so it's decent enough.
As for wildlife, we saw turtles, trigger fish, banner fish, clown fish and plenty more!
I second Quero, def go straight for your open water course, but be warned it's so addictive!
