A couple ideas. I would suggest that, on the trials and testing the ladder, a) you put 5 steps on the ladders, and b) the last step should have a reasonable weight attached.

The problem with a light ladder, obviously, is that it doesn't go down, but tends to drag on the surface, particularly if there is some boat motion.

The problem with a limited number of steps, is that you need to get your feet under yourself. If, for example, your feet are at or near the surface, and assuming you are strong and have a really good handhold, the principal force against the step of the ladder will be away from you, not vertical, which makes it very hard to get up. On the other hand, if the ladder step were two feet down, for example, you can be in a crouch position, and lifting yourself out of the water is much easier.

Building a longer test ladder gives you the option to test this theory and figure the right weight and length.

Peter