Tiadude (if you're still reading)...I'll throw gasoline onto this fire by trying to direct the discussion back to information relative to your concerns.
You've probably made your decision to do a liveaboard trip based on the following advantages:
1) Unlimited diving
2) Ability to move to different locations to take advantage of best dive conditions
3) Proximity to the atolls without many trips on smaller vessels
Nothing at any shore-based location compares to being able to come and go as you please to do your diving. After a "check-out" dive, most liveaboards let you and your buddy dive whenever you want (including night and early-morning), leaving the responsibility of nitrogen uptake on your shoulders. That's why almost all offer NITROX, so experienced divers can maximize bottom time. You'll see camera tables with tens of thousands of dollars' worth of gear. Serious, experienced divers go on liveaboards to spend every possible minute underwater, almost always to shoot photos or video. If you don't have much dive experience yet, don't worry, they'll assign a diveaster to you until your comfort level improves. The big boats in Belize tie up to moorings at some of the best sites in the country, often only moving once a day, and then only a few hundred yards. The dive guides and photo pros, almost all Belizean, know where the best photo and video opportunities are. Even if you're not filming, just seeing the best diving in Belize for days on end--and you will--makes it a memorable experience. Although you will be given flexibility in choosing your dive time, there is always a divemaster on deck keeping track of divers in the water. Every diver is logged in and out. Your safety, enjoyment and preservation of the reef are their priorities.
Large oxygen tanks, cell phones and VHF radios with sufficient range are onboard to help in handling emergencies. There is a recompression chamber in San Pedro. Make sure your DAN insurance is up-to-date...it will cover many in-water emergency costs, although costs for basic services at the hyperbaric facility are included in your trip expenses.
Since your trip is planned for March, information about how weather affects diving at the atolls of Turneffe and Lighthouse is relevant here. It will not be hurricane season, meaning discussions about errors in judgement in the face of a severe, unpredictable natural disaster don't apply. While March may bring the last of the "northers" to Belize, your dive crew will know about it days in advance and plan your dive week accordingly.
You will hear that your money doesn't go to the country of Belize and you won't see any Belizeans. All liveaboards pay hotel tax to the Belize Tourism Board. They all hire local crewmembers, buy fuel, parts, groceries and supplies locally. The van that takes you to and from Goldson Airport will be driven by a Belizean. Since the regular trips are from Saturday to Saturday, you'll return to Belize City on Friday and be given a choice of doing a ruins trip, a zoo trip or another local tour, all using a local operator. Your departure tax will go toward preservation and development of Belize.
Most people that dive liveaboards plan subsequent trips so they can maximize their dive time at the premier dive sites around the world, just like you'll be doing in Belize.
Enjoy your vacation!!!