I'll second the recommendation about Conch Republic Divers in Tavernier. My wife and I have dove with them two Januarys running now. They are very service- and safety-minded.
In '05, we requested Spiegel Grove when we reserved, and they took us ... just the two of us ... a very long trip for only two divers. (A great dive, by the way.) Our other dives with them over three days included the Eagle, and several very nice shallow reefs.
Last month, the conditions were pretty bad with sustained southeasterly winds. They are not shy about cancelling dives (full refund, of course), though they also have their grey area where they will take experienced divers out who want to dive despite realistic description of challenging conditions. We did two trips like that, and Captain Jim handled the trip, the mooring, and the reboarding exceptionally well. However, the shallow reefs were badly stirred up, and we decided to cancel our last day. Again, because the conditions were iffy, even though they went out, we received a full refund for those dives. You won't find that everywhere!
--Eric