I live on a river and built part of my dock using treated 4 x 4 post sunk into the sandy bottom about 3 foot. I want to add more docking and thought I'd ask for opinions on using treated lumber for the supports. How long of a life span can I expect for the posts being submerged year round? our river doesnt freeze over in the winter. The 4 x 4's are a standard treated board purchased at the local lumber yard.