Sam Hopkins wrote:

I've built a dock onto existing water like that. I used pressure
treated lumber and contruction blueboard as the floating material. I
built it exactly like a deck. I used 2X10's as the side boards. The
blueboard was 2" thick and I used 4 pieces together for a 4X8X8"
float. My walk out is two sections and each section is 4'X12' long.
Each section has 1 4X8X8" module of blue board keeping it up. The end
section is 10'X8' and has 2 blueboard modules on either side. To keep
the dock from moving side to side while you walk on it I put U bolts
in the wood on the side sections and inserted 1" galvanized pipe
through them and into the water and then pounded them into the mud at
the bottom of the pond. It keeps it rock solid. Here's a link to it.
It only shows one walk out section, I later added a second.

http://www.wildlifereserve.org/dock.html

Blueboard should hold 55 pounds per square foot of foam.

Sam

That would be 55 pounds per cubic foot of foam, not per square foot. A
large difference.

Jon