Hey Matt, you might want to look at my website and how I built my dock using reinforced 4 inch pvc pipe, it is only 16 ft long but is sturdy as a rock and will never rot. Might give you an idea.
PS - Make sure you get it high enough above where you figure the water table to end up. I had to add some couplings and 9 inches of PVC to keep to wood out of the water this summer!
Bob
