I've think I've come up with a fairly easy solution, which incorporates Mary's hand loop and my three-point foot loop idea. I've tried it (in calm conditions), and it works very well, with almost no upper body strength required. Hopefully it will work just as well in windy, wavy and panicky conditions.

Start with a rope about as long as a lower bridle wire, tied at the bridle shackle, with a carbiner at the other end. Then tie a knot forming a loop onto the bridle wire, about half way down.

I tied it on the bolt to prevent chafing the rope.

A simple over-hand knot forms the hand loop.


After righting the Wave, tie a knot in the righting line to form a loop near the middle, and clip the carbiner to it.


Then you can step onto the righting line (it won't move now) and hold onto the hand loop, and lift yourself up onto the hull or crossbar.
When the boarding line is not needed, you can clip the carbiner to the bridle wire or bungee it to the bow eye. I used bright line for contrast, but black rope would be inconspicuous.

It works so well, I can now see using the Wave as a swim platform on the no-wind days.