A question that is asked many times- about any number of places.
The Bay Islands bear the brunt of this perception because, up until very recently, they were the last hold-out in the ongoing decimation of the Caribbean. There is still a lot to see, but not as much as last week, nor last year, nor as it was in 2004.
The area was heavily fished, and in terms of the Caribbean, this occurred very recently. Curious that very few ever pose this kind of question about Jamaica, or any of the other islands that were invaded in the 1950's. There are many places in the Caribbean that appear like a lunar landscape underwater, yet divers still show up.
So you wanted to know the quantity of fish. Enough to make it worth your while.
So, all these soft and hard Corals. Yep- I agree, not as exciting as a fish. But- what is hiding within those Corals? What creatures use this, what is regarded as the Caribbean's largest variety of Coral... what is hidden, right there in plain sight?
Any number of divers are frantically searching for their big Barracuda, a monster Parrot Fish, they're eyeing up the Lobsters. Utila and the Bay Islands offer something beyond the obvious. Slow down, be observant, stick with your DM.
It's obvious that any other advice you will get will probably not cause you to cancel your upcoming Spring trip- so use this to your best advantage. Look beyond the common and obvious. Someday, and unfortunately not too soon, you'll be able to say, "Utila? You shoulda' seen it back in 2014!"