If you have a local quarry, Since you are scouts, how about designing and laying out an underwater compass course in your local dive quarry?
Seeing how "compass courses" are taught above water for all Scouts, seems like an idea for below.
Our local quarry has a compass course set up with 9 points on it. Each "point" has an anchor, float and a shape on the line at a certain depth.
The shapes are all made out of PVC and about 5 to 6 feet in size so that you can swim through them. They are in the shape of a square, diamond, rectangles, and I think circles. (I'm not positive on all the shapes, but you get the idea). Put different shapes in different places and they are easily identifable by their shape.
When setting the course out, you'll have to do some precision underwater measuring and angles to get everything laid out according to your plan.
If you've got varying water levels, you can make an anchor system out of two different size PVC pipes running vertical from the anchor with one inside the other. You put a small weight in the inside one and let is slide up and down inside the larger one. Affix the rope to the inside one. (note the inside pipe might be 1 foot long and the outside 10 foot long to give about 9 feet of travel.)
You can make this as big or small as you want and develop all kidns of new ideas.
I'm betthing your local dive quarry would pay for the funds for the pipe, rope, anchors, etc since their customers would benefit from it.
Other service ideas at a local quarry are run "cave line" from all the sunken items to the other items and then map out the quarry. Adding signs zip tied to the lines is a neat idea, but you'll find they won't last long by either getting broken or taken.
hope that helps.
