I have been to Bonaire many times, and just led a group of 16 to Habitat in December. Bonaire is very arid but certainly not volcanic. It has a fringing reef system and I am not aware of any muck.
The macro shooting is fantastic and nightdiving is oustanding. The house reef at Captain Don's is one of the best (easy navigation with the divers highway), but there are many really great sites to choose from. The house reef runs between 25 and 130 feet deep and you can dive 24x7. Great people - Let the DM's know what you are looking for and they will do their best to find it for you.
You can also rent a car/truck and shore dive till you drop. Site are well marked from the road. There is also an operator (I think his name is Larry) that will take you out on the exposed side of the island if you want a little higher adventure.
Since 9/11, diving the town pier or salt pier require a bit more planning and can only be done accompanied by a local DM with a max of 4 divers per DM, and as always, requires prior permission from the harbor authorities.
Have a great time, and let me know if you have any other questions.
Cheers,
Sieg