Boat Ladders
May I respectfully suggest you look at
1. a West marine catalog (paper or on-line) or the chandlery on this very website or any other chandlery or that matter
2. do a Google search on "boat ladders"
3. look around your marina
I think the reason Roger and I have taken this tack is that boat ladders are so ubiquitous that the question really comes out of right field at us. It really has only a few simple answers, and the few varieties that are available have been pretty well documented.
Aft ladder off your transom, usually built into the stern pushpit; or hanging ladders from either the transom or the side of the boat.
EDIT: It seems you already have a ladder. If so, may I guess that it gets too bouncy at the stern to use comfortably? If so, buy a ladder you can use at the side of your boat.