In my neck of the woods, the posts are made of steel pipe with a flat plate welded to them and set on the bottom. The point at which the dock attaches to the pipe is adjustable to allow for changes in lake level or for any settleing that may occur.
If you do need to set them in concrete, it is a lot more difficult, but I do know that concrete will set up underwater, as long as it is held in place with some type of form.
Good Luck!
Dodgeman