Mine are not in that deep of water and not buried that deep, but the holes are flaired and the concrete fills the flair. I got the idea when I could not pull some very old cedar corner posts on a fence line using a tractor (in every way possible). When I dug them up, they had short cross pieces notched into the bottom. Using the natural strength of the soil really works. However in 10 feet of water, a floating dock might be longer lasting. I'll post some pictures of my combination fixed and floating dock when my son figures out how to load up the picturs(and he will).