Hi dpace,
Been a couple of times.
IMHO, the only really decent diving in Venezuela is in the Los Roques archipelago, which is best done on a live-aboard. It's quite good, but not world-class. Some of the closer islands are a long boat ride from Margarita Island, but can be arranged. Some ops do a night or two live-aboard. If you are thinking right off Margarita Island, forget it (poor conditions, high bacterial counts, some unreliable & unsafe ops) & just enjoy the good restaurants, discos, casinos & thong bikinis. Especially avoid an operator called Octopus. If you must dive right off the island, go with International Dive Center. Run by the best DM on the island, it's been around a long while. No website or toll free phone. Look it up when you get there.
Enjoy.
DocVikingo