I think it has to be on the transom.. Anything on the sides might be too tippy to actually climb. It's very hard to climb into a cockpit that is tilted towards you. 2 steps underwater is for me the minimum. Handholds help a lot.
Something like this:
Check out our 4-Step Over Platform Telescoping Swim Ladder and more from West Marine!
www.westmarine.com
There is a 2 step version too. A lot depends on where you will mount it. Measure 3 times!