Safe Boat Hull Cleaner?

I am looking for a hull cleaner I can use while our boat is in the water. Many times during a weekend we anchor and jump out and swim. This would be a perfect time to scrub down the hull of the boat. The problem is the cleaner needs to be safe for the creatures living in the lake. Can you give me any suggestions on a brand and whether you use a sponge or rag to do the work? Thanks