The boat itself is actually not the correct type of boat for scuba diving as it sits very high on the water, which means that when you go to get in or out there's a significant drop and distance between you and the water. I found it very difficult to get out of the water with all my gear on and the crew, despite my polite (and desperate) pleas, wouldn't come down the ladder to help me. As a result, I was slammed against the boat three times and still have bruises on my left leg two weeks after my trip.
I ordered and paid for all gear (including wet suit, regulator, etc.) for a trip to Molasses reef. The crew member who assembled the equipment forgot my wetsuit and when I mentioned it he muttered and swore at me under his breath, and did nothing. As luck would have it, another diver on the boat was listening and had been before with Horizon and knew that they had spare gear aboard. Wetsuit problem was solved by a customer.
Lack of safety was the most alarming thing on board with Horizon. Nobody checked our gear before we went into the water, and when I recognized a blockage with the air in my tank (which meant I could not inhale deeply - pretty major right?!!) they were dismissive. I battled with small breaths for my first dive because I really wanted to go out, but I'd never do that again. It was simply unsafe.
There was also no safety discussion in the briefing (the briefing focused on you-break-it you-bought-it, if you're not back in 1hr we'll leave without you, etc), so I was not aware of things to avoid. When I accidentally got a little too close to some coral, it stung and hurt like crazy. Another diver told me it was fire coral and the crew did nothing. A concerned diver told me I needed treatment and pushed the crew to bring out the first aid kit which contained a spray. The diver then applied the spray to my leg, which by that stage had become red and inflamed.
I've dived in Thailand, Bahamas, Malaysia, The Great Barrier Reef (awesome if you ever get a chance), Mexico and now the Keys. I've never had any complaints with any of the dive crews I've gone out with as they've always gone out of their way to make every dive safe, fun and comfortable.
Unfortunately, Horizon only seems to care about the fact that you paid. There's no passion for the reef like you find with most dive operators and the lack of safety was alarming.
Google these guys and check out scuba-forum - there's a very disturbing article dated 20 Jan 2007.