Depends on how thick it is. For me, if I dive in a skin, I use about 8#'s. With a 3 mm full suit, I have to add 6#'s with a 2pc. 7mm or dry I have to add 24# to that original 8#'s. You don't have to worry about the 7mm, unless you plan on going deeper than 50'. It's not an exact science, since everyone's body compositions are different. The best way we've been able to come up with (because you'll most likely have a full tank) is to get close, empty your BCD release your breath (reg in mouth of course) & if you just barely begin to sink, add a couple of #'s.
You may want to call ahead & try to reserve what you may need to rent. I'm not sure if they do this or not, but some weekends they're really busy & if you don't or can't reserve, it's first come, first serve on the equipment there.
For the treasure hunt, it looks like we'll have a banner up for our club. Look for us there. If nothing else, I'll be driving a silver '99 Toyota RAV4 with dive flags on the windows & front plate.
