Freshwater Sponge

Dale noticed a tentacled thing reaching out from the corner of our dock. The elongated arms would sway and float with the waves and currents.

I placed a frisbee under it to capture a clear picture of its structure.

Without the frisbee, it was hard to get a good picture of it from above. It looked like a tentacled monster from the deep.

It's actually a freshwater sponge, with some algae cohabiting with it, giving it the greenish coloration. It filters particulates from the water, giving it sustenance, and keeping our water clear. 

There is only one family of sponges that live in freshwater in the U.S. - the Spongillidae. You can see the coarse texture of its surface in this closeup

Here are some pictures taken from underwater: