For getting out of the water after a swim, I've got something similar to this:
www.bcf.com.au/p/blueline-aluminium-boarding-ladder/M115679.htmlI think it probably came from Whitworths (the BCF one has terrible reviews) but their website is no good at present.
It folds and goes in the rear compartment, and when needed hooks over the side. Not an easy climb in full kit when the boat is underway mind you.