In our ocean study, we found a non-ficiton book about sponges. I can't remember exactly what it was called, but if you go to the ocean section at the library, you will probably find something sponge-related that would work!
We cut up some sponges and used them to stamp designs on these "sponge shapes." The kids thought it was great fun--which led us to using the sponges to paint underwater seascapes as well.
You can paint anything with sponges--and it was easy to set up and fun for the kids.
Have you tried it before? Did your kids like it?