Hi JHenthorne
The Maldives are all islands and each resort is on its own island with pretty much all of them (at least those who cater to the diving community) having their own dive operations. Because of this, there isn't really a true "land based operator" in the Maldives - they're all part of the resorts.
You're best to look for an island that has good offshore diving reefs, as well as the dive boats if you're wanting more than 2 dives per day. Typically, the dive boats will do a morning and afternoon run (i.e. 4 tank dives per day) and a night dive if they get the numbers - weather dependent of course. So a resort with some good house reef diving will be your better option if you want more.
We stayed at
Kuredu. Book villa type accommodation, not the bungalows as this will give you a better position on the island. (Bungalows were hidden around the back on the not so good side)
Diving was great. The Dhonis (dive boats) were comfortable and good to dive from. Everything is taken care of - no lugging dive equipment around here, unless you really really want to!
Other places we've stayed at in the Maldives have been Kanuhura and Reethi Rah - part of the
One&Only chain. Reethi Rah had the best diving op of the 2, although things may have changed on Kanuhura because of the rebuild required after the 2004 tsunami.
We preferred Kuredu as it's more diver-centric and better value for money.
Look out for "hidden" costs like meals and extras. Because each resort is on an island, you're basically a captive! Take as much as you can so you don't have to buy anything in the shop at exhorbitant prices. Also best to buy either "half" board or "full" board as this will include some of the meals depending on which option you take.
Have a great time! The UW life is fantastic. We saw lots of big stuff - mantas, rays, sharks, dolphins, turtles and the reef for the smaller creatures is superb. Lots of variety too - drift, deep, channel, (Strong currents = sharks!), wall, wreck - as well as the standard reef dives.