Sorry I don't have a picture, but I've seen a couple of different basic types:

1 Oil drums are placed inside a band of pressure treated lumber which is then decked over. There are also plastic drums built specifically for this purpose.

2 Concrete is poured over a mold containing a very thick styrofoam plug. In this case the styrofoam is @ 12" thick.

I keep my boat at a floating dock. The dock itself is of the styrofoam/concrete type, which is anchored around standing concrete piles. The dock is thus very stable and rises and falls with the tide, making docking very simple.

Hope this helps.