This is a very honest review of Koh Tao.
As an experienced diver based in Indonesia, I found the diving in Koh Tao completely underwhelming. I only did four dives during a weeklong stay, and that was at least two more than I needed to get a good picture of the diving scene in Koh Tao. I will caveat this by explaining that the visibility was poor in late October, sometimes down to just a few meters. The four sites I visited were Red Rock, Green Rock, White Rock, and Twins. The most interesting part of the dives tended to be when we headed up for our safety stop, and there were hundreds of Christmas tree worms, which were not at all disturbed by me getting up close for a good look, unlike those in Indonesia that tend to hide if you get close. I mostly encountered other divers underwater, with instructors hauling DSDs directly overhead or large groups of new divers cutting directly through the middle of our group. Every dive site was crowded.
In my experience, the dive operations are a bit chaotic. We boarded the enormous wooden boat from a small tender that left the beach in front of the dive resort, and once onboard, we would go upstairs for a boat briefing, then down to set up our equipment, and back upstairs for the duration of the cruise to the dive site and a site briefing. Fortunately, the boat was far from full when I was there because I definitely would not have joined a second day if it were at capacity. The dives were all led by inexperienced, newly-certified, mostly American divemasters. As such, the dive experience varied between guides. The surface interval was spent on the boat, with cookies, tea, and coffee available. The chaotic logistics began after completing the second dive, when we headed for a major pier (we were not forewarned and had not brought shoes). The boat then tied onto another boat and we were instructed to form a chain across several huge boats to pass the bags of dive equipment to the pier. This was very unexpected. Once we climbed through several boats, we disembarked on a rusty pier and walked to a parking area, where local taxi trucks picked us up and delivered us back to the dive resort. All of this was fine for me but would be problematic for anyone with mobility issues.
I visit Thailand regularly because I love the food. Koh Tao did not disappoint on that front, and I savoured dishes like laab moo, the best masaman curry I've had, a Southern attempt at khao soi, and a variety of tasty western dishes at the restaurant located at our hotel. I spent most of the time working on my laptop in my room or at the beachside restaurant, so this was a cheap destination that suited my purposes while my friend was out diving daily. It was relatively easy to get to, flying to Samui via Singapore, then booked a taxi and ferry with Lomprayah at the desk at the airport. There were ATMs at the Samui airport, as well as a money changer. The ferry was not that comfortable, with hard plastic seats, and we had to carry our suitcases to the front of the ferry and then collect them while everyone else was trying to do the same and disembark. On the return, we booked a catamaran instead, and it was much more comfortable and organized, although the boat swayed more than the ferry (for those who get seasick, the ferry would be a better option). Koh Tao was almost entirely cash only, which caused some problems for us because the ATMs ran out of cash midway through our stay.
Overall, I can see why Koh Tao is so popular with young backpackers looking to party and try scuba diving or get certified. I recommend skipping for experienced divers, mature travelers, and those with higher budgets.