My wife and I are going to Bonaire March/April 07, so I'm listening in on this thread as well.
We've done one 2 tank boat dive when we were on Maui, and it was an experaince! We had mesh bags for our mask,fins, snorkel, camera, etc (basically anything "allowed" to get wet. We also purchased one of those "roll down" dry bags, this was very useful, we put our towels, glasses, and log books in it.
The BIGGEST mistake I made (yea, I'm a dumb@ss sometimes too), was that I put my towel under my seat while I was getting my gear all squared away. I had intended to put it into the dry bag, but being our first boat experiance, I got sidetracked... By the end of the diving when I went to reach for the towel, it was SOAKED, just like the captian and DM's had said it would be.... Well at least I didn't really need it much, we hung it off our balcony for the next couple days to dry
BTW, LISTEN to the captain / DM / Crew on the boat, they will most likely tell you where to put ANYTHING that needs to be dry, assume that ANYWHERE else will be WET!
Other than that it was a great time, though by the end of the day on the way back to shore I was starting to get a little sea sick. Fortunatly, all I needed was to watch out in front of the boat, and I was fine... A good indicator I think is do you get sick if you try to read in the car? Some people can read a novel in the car, me on the other hand, if I try to read 1 sentance I start to feel a little queezy.
I think next time I'm going to get some meds....