Non-rigid ladders are next to useless on SIBs or RIBs. They swing up under the boat as you put weight on them, and you spend your energy trying to stay out from underneath, rather than climbing up and over the tubes.
Haven't tried Nick's suggestion, though I've seen them advertised. It seems it would be better than a rope ladder (then again, it couldn't be much worse, I suppose...)
Someone needs to come up with something that braces a collapsible rigid ladder on the underside of the hull. Sounds easy enough