One thing about exposed styrofoam is that muskrats burrow in it. Makes a mess and eventually will sink your dock if not replaced, and this is not easy to do many times. Plastic barrels work better. If they leak you can replace them by partially filling the new barrels with water, putting them in place and using an air pump to displace the remaining water. You have to orient the opening to put the air in at the bottom or create a new one with a drill and plug it after displacing the air.
