When I said that I imagined it was like Indonesia, I was thinking that fish can be quite similar across the whole Indo-Pacific because suitable habitat extends right across the two oceans. But I suppose Hawaii is a little colder, and doesn't have the coral. Perhaps it's more comparable to the Azores. Except that the fish must be completely different from those in the Atlantic. The special thing about the Azores is not the regular diving off the shores, where you will see some fish sure, but a couple of seamounts where you can see eagle-rays gathering. So I am thinking about things in Hawaii that I can't see elsewhere.
I am worried that I might be disappointed by some of the dives, given how much they cost, and that I've dived a lot in warmer, more coral-rich places in the Pacific. And it's frustrating that I seem to be expected to book the dives up weeks in advance, so I can't try one excursion and see how much I like it. Maybe I'll just book a day or two before I go, and see how it works out when I'm there for more. Diving independently, as you suggest, would be good, if I can find someone to dive with.