The "mile deep" dropoff is an old, lame Divemaster's tale.
While this is surely a bit pendantic, the drop-off is "only" to ~1000ft, and you have to go out around a mile further offshore before it gets to 3000ft. Here's a map with depths (in meters). While this information may be new here it has been well known on the USENET discussion groups for years.
Grand Cayman's what's expensive & overpriced. Just get out, or jump over to Little Cayman or Cayman Brac, where the diving is generally considered markedly superior...just make sure that your trip is long enough to amortize the extra ~$175/pp-RT airfare cost and shop around for off-season "2 for 1" promotions and the like.Unfortunately, the Cayman Islands are an expensive destination by any standard. The hotels and restaurants are clearly aimed at the high end of the tourist market. There are no "cheap" places to stay. For the less expensive places, check out Eldemire Lodge, Admas Guest House and the Comfort Suites.
Suffice to say that after 2-3 dives on St. Maarten, I stopped bothering to carry my UW camera with me...Sorry, I cannot comment on St. Maartens diving.
-hh