Some aquatic plants from Creatures 3 to address the lack of standalone aquatic plants. Not much to say about this, really.
Changes from original C3 sponges:
-Plants are pushable and pullable, seeds are edible
-Opal Sponge lives longer, but is less populous, than the Orange Sponge
-Spore behavior depends on the creature-to-sponge ratio: More creatures than sponges will make the sponges produce spores that fall to the bottom and sprout near immediately, while stable populations will produce floating spores.
-Sponges are less stingy about where they grow.
You will need C3 installed for these to work properly, I'm afraid.
CCSF 2012 Update: Sponges no longer have teleporting seeds and no longer fall through floors.