Funny I asked about bladders in another thread and then found this....
You might check at a swimming pool supply store. There are some heavy coated water bags with fill caps built in that are simply used as weights to hold pool covers in place. They hold about 3 gal each and can be stacked. The would form to a variety of cavities or nooks. They are about 2' long, 10" wide and about 8" high when filled, but they could hang over a rail or stand up if something were holding them in place.
I think they would be easy to clean out as well. :snorkel: