A step up
When we are at a floating dock, our deck is sometimes four feet higher than the dock. I can climb up, but my wife needs a step up. I made this little single step to hang off the shrouds where she can also get a good hand hold. It’s all we need to climb aboard when the deck is way up there.
I have also seen a “fender step”. This is a small boat fender that has a step cut into one side of it. You also hang it from above. The bonus of this items is that you have a dual purpose for it, it can also serve as a boat fender in addition to the step.