Spongey Tentacles

There was a tentacled creature reaching out from the corner of our dock. Its 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 looked like this scary-looking, tentacled monster from the deep

It's 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