I did a blackwater dive with Big Island Divers last Friday and highly recommend the dive - it's a unique, weird, kinda freaky, but awesome experience.
We saw dozens of popcorn-sized Hawaiian bobtail squid, and had larger squid schools dart between us (one inked right by me and another bounced off my wife), saw a few seahorses, and a lot of comb jellies and other strange critters.
I brought my Olympus TG-5 with a focus light and a single Inon S2000 strobe and it was *very difficult* to get any good images to come out. I think I'll need a few more of these dives before nailing some awesome stuff - the biggest challenge was getting focus. I did get a couple solid shots of the bobtails and seahorses though, so it wasn't a total loss, but nothing like the big beautiful macro blackwater photos pros take.
The TG-5 doesn't have a good rear focus / thumb focus mode, and if you go into manual focus its really easy to bump it out of manual focus again next time you take an image. I had the best success putting my hand near the critter I was trying to shoot, focus-holding on my hand, then moving my hand away and snapping the pic.
Basically, definitely go on a blackwater dive, take photos if you want, don't expect amazing results the first time and you'll be happy you went.
