About 4 years ago, I built my dock dog ramp from a 2'x4' plastic table top (costco), 2'x4' outdoor rug (homedepot), some composite deck wood (homedepot) and stainless hinges/hardware (homedepot). It works like a charm and I can tie it up out of the water to keep it from growing in the delta water. However, I would not recommend this for a boat, just a permanent install on a dock.
My neighbor has this one. It worked but they haul it in/out every time and it fell apart after a couple of years. http://www.yachtsee.com/doggydocks.htm