I ditto what Parrothead said about the boat captain. They are JUST as important if not moreso than the divemaster. Don't underestimate thie importance and how instrumental they are from a safety standpoint. The boat captain is also responsible for the everyday boat/engine maintenance and making sure that problems with the engine are addressed immediately.
When I need to rent an additional boat(s), I am very selective about the boat. Before I ask "how much" I ask who the captain is. Of course the boat I normally lease as a second boat is a great boat with a great captain, as is my 2nd choice boat.
Recently I had a last minute emergency and had to rent a boat with a captain I was unfamiliar with for a day...I will NEVER rent that boat again! I will literally cancel a trip before I rent that unamed boat again! Poor Jayjoans and his group know what I am talking about and they were amazing sports about it!
There are only two or three boats on the island that I am comfortable/confident with when an extra boat(s) is needed.
Tony is selective as well, and he makes arrangements with reputable shops or boats when he has a group.