So many questions
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In Rangiroa, I went with Six Passengers. Pretty OK, but a bit "factory" to me. You are diving almost each tim with another dive guide. I do not believe it is a good idea. Next time I would try to choose a "smaller " structure. They have one dive where the "attract" sharks with a small box with food. But they do not feed them. Just attract them. It allows you to see sharks @ 20 meters that are normally WAY DEEPER USUALLY.
For Fakarava North, I dived with Dive Spirit. Would go back there without hesitation.
For FAkarava South, Tetamanu is not cheap. I believe that ther is another one, but I doubd that this would be better or much less expensive. You have to realise that their degree of isolation is TOTAL. They practically have a monopoly on the pass. They are ideally located. Expensive yes, vey much so but I would get back here in a second.
When it comes to rental, this may be possible but I am not aware of it. Importantly, the interesting bits are the passes and the aouter reef. There is just no way one can do this with no security at the surgace. In the ragiroa main pass, there are up to 7 different currents that can push you, in a few minutes, very far at various places. I really doubd, that anyone will let you do this.