Yes the diving is not fantastic but worth getting wet. The dynamite fishing really messed up what could be a phenomenal dive spot. There is some really good diversity as the area is in the coral triangle. I've dived there when getting in beach vacations with my wife. The dive shops there tend to most their business in C-cards and when I'm in an area like that I like to use the smaller shops so I might have a better group to dive with or be on my own with a guide. I used Mango on Gilli T and was the only one on most my dives. On Gilli Air I use WWW diving and again usually end up alone with the guide and picking my dive site. If you use a busy shop I'd definitely go with Tippy's ( Hi Tippy!) recommendation and hire your own guide so your diving isn't ruined by a bunch of inexperienced divers.
As for sites all the sites on the north of Gilli T are pretty good,but the coral tends to be monoculture, but often overlooked in that area is the shallow ledge along the island. It's shallow but holds the most diverse and profuse life there. A couple of years ago my guide and I finished up our dive there ( a very rare 85 minute one, which he got in a bit of trouble for, needless to say he got a good tip) and it's one of my favorite spots. My other favorite sites are Hans reef for some good macro, last year the two times I dove it I spotted an ornate ghostpipe fish and two pairs of Robust. The Bounty Wreck is also nice where you get to see some of the diversity of the area.
