For anybody in Bonaire or about to head out, we discovered that CIEE in town are holding free Fish ID lectures on Monday nights, 6.30 - 8.30. We only went to one and had to dip out early as we hadn't realised it was two hours, but we learned some interesting stuff. It is pretty in depth and I think the lectures must build on each other (ie they are not a repeat every Monday), so you won't get an overview of what are the main sorts of fish etc, but if it's like the one we went to, you will get some examples of different varieties of the same sort of fish and how to tell them apart. A different thing to do on a holiday night out!
It takes place in the CIEE building, which is in the north of town, (near the pizza place?) and has a big sign outside with a ashrimp on it!