The following is what I learned about carving pumpkins underwater, for a crowd.
I should also point out that sk8ter20art posted a great video in his instructable carving a pumpkin underwater.
So hopefully I can add a few tips and tricks on top of what he's already gone over.
Please bear with me with pictures. I had a little more time day one to mess around. Day two the aquarium was understaffed and I had to help out with day to day chores while getting prepped to do my pumpkin. So there's no good set up shots for the second pumpkin. I was also solo on the first day, so there's no good in processing shots from that day. When you're underwater and on someone elses time, you kind of have to work with what you have. Hopefully everything makes sense though.
Step 1: Create your Design
Also, remember where your pumpkin is going to go. I was carving for an aquarium, my signature batman symbol wouldn't really cut it in the underwater world.
For my first pumpkin, I decided to copy something someone else did. I did a google images search for "fish pumpkin" and on the first page I found a great Angler Fish.
For my second pumpkin, I did some research first. This specific aquarium has a lot of Seven Gill Sharks, and that's something a lot of people enjoy seeing. I decided I was going to try to figure out how to make a seven gill (since I couldn't find any designs online). I found a good greyscale image of a Seven Gill here. I pulled the photo into Photoshop, selected just the shark, and filled it with white (I planned on just taking the orange skin off of the pumpkin. I created a background layer and filled it with orange (to see what it'd look like on the pumpkin). I drew in a few extra details, and I was happy with my pumpkin pattern.
I've gone ahead and attached the photoshop file in case you'd like to see the process.