Because of the number of dives we have, we're always the first group off the boat with Lahaina Divers, thus wind up being the last divers back on-board as we're excellent on air. For the most part, our experience has been that they'll let you dive your tank, however, you're required to stay under the boat to do so once the "tour" is done.
The tour is the biggest reason I don't go out much with them anymore. They have a set route and timing they follow to make sure that they're back under the boat in about 25 minutes. I understand it is because they have a lot of beginning divers who suck their tanks, but it isn't my style of diving, which is why I dive with an operator who better matches my style of diving.
Hawaii is a very different place to dive. IMO a great dive guide makes all the difference in the world in your experience and in what you see. The topography in Hawaii is lava, rocks, and hard coral for the most part, and there are some amazing endemic creatures that you cannot see anywhere else in the world....IF you take your time and know what you're looking for. Hawaii is also a place where going slow is paramount to seeing the creatures that are hiding out on the walls and in the reefs.
I'll never forget one day when I requested to dive at my favorite Maui dive site (Red Hill) and while the boat was heading there, hearing a diver remark that he might sit out the dive since it is the worst site he's ever been to. When I asked why, he said there's nothing but sand and rocks, and except for the turtles at the start of the dive, the place was dead. I asked who he had been diving with, and it wasn't the operator we were on. Since he was in my dive group, I encouraged him to dive with us and told him that a really great DM made all the difference in the world, and that I thought he would have a far different experience that day.
He agreed to go (mostly because he didn't really want to sit on the boat) and when we returned to the boat a little over an hour later, he couldn't shut up about what an amazing dive that was, all the critters we saw, and how glad he was to have gone. He left a big tip for the crew saying he completely understood how they made all the difference in the world in his experience.
IMO Hawaii diving requires a great guide, and although I'm an excellent critter spotter after so many dives on Maui (nearly 200), it's so much better to dive with an ever better critter spotter than myself.