I have been looking for the right boarding ladder too. I don't like the idea of putting holes in a boat.
I have looked at a few but am waiting to win a lottery. A portable ladder is preferred. There are a couple that I like. One has the big hooks that can hook on the gunwale on either side and the other is a rope ladder that can hood onto a cleat.
Until i get around to buying a ladder, I use a very inexpensive yet functional reboarding device by stringing a line from the front cleat to the stern cleat with just enough sag to allow someone in the water to step on it and then reboard. I use braided lines that are easy on bare feet.
If/when I get a real ladder, I will repost with the details.
Mike