Dwight,
Just a couple of thoughts, though I bet they have already crossed your gunsights.
I presume the ramp will be hinged for vertical changes as the water level varies. Will there be adjustments required at the length of the diagonal stand-off brace since it will be dropping (or rising) at a different rate than the main ramp?
All the tubing ends will have to be welded shut to keep out moisture. I was once told by a trailer manufacturer that this is why channel is preferred for their product.
When I saw your diagram, I did a double take. I am already thinking about a nearly identical design as an addition to the deck we are already working on. Then I start looking at used 18' pontoon boats.
