Dog swim platform
Dog feet aren't very well-suited to ladders. What pops into my head (ooch!) is a sort of swim platform that the dog can scramble up on, then climb over the tube.
First, get some PolyMax kennel floor— search for the 'Games with Frames' thread which has photos and sources. Also, search PolyMax for further posts on the stuff, including Laura Howe's dog-friendly boat frame.
Second, cut a 4' x 2' panel of PolyMax in half = 2' x 2'. Get some aluminum flat bar stock (local hardware) and cut two pieces to form an X under the PolyMax grid (adds stiffness for a big dog) and round off sharp corners. Drill holes (say 3/8" at the ends). Then rig some non-stretchy (i.e. polypro) rope through the holes in the bar, around the PolyMax grid, and up to another piece of bar stock or hardwood (carefully smoothed) with holes 2' apart. The outer pieces of rope will be longer than the inner pieces, so the platform is just below the waterline, level or inclined somewhat outboard so the dog has an easier time scrambling up.
Rig it to your frame with straps or whatever, perhaps at the stern where the pooch won't get conked by an oar. It should flip up onto the boat for running rapids.
I'm imagining this, so far. But I might build one for my waterdog, Ruby, so she can get back into my big cat without help.
Worth a try, p'raps. Good luck.
Chip