Hey Sarah,
There were several recent discussions on diving in FP including Rangiroa. Synopsis: 150' + visibility, great marine and coral life, amazing cobalt blue waters, a lagoon that is so vast it seems like a waveless ocean, and weather that is awesome (in August when we went this past year).
There is only one full service resort the Kia Ora, and one small Novetel (11 rooms I think). Two main dive ops, Top Dive ( my recommendation) or the one at the Kia Ora. FYI Top DIve is a 100' walk down the beach from the Kia Ora.
The Kia Ora is a good hotel, with good to very good (not great) food and very good service (for FP). Spend the money for a beachfront room as they have less mosquitoes than the garden rooms. Overwater rooms are inconvenient at any FP resort. The novelty wears off in one day when you find yourself walking back and forth on the gangways.
Rangiroa is like falling off the face of the planet. THe only things to do are dive, dive, dive, read, suntan and relax. The bar is empty at 8:00 at night so don't expect any nightlife. I would recommend spending 3-4 nights in Rangiroa and also a couple in neighboring Fakareva (10 minute jump flight).