You're looking for what's called a Hoyer lift, something like this
. It requires a sleeve be mounted to the dock, usually bolted on top to a structural member. Can also be flush mounted below the surface of the dock. Simple hydraulic ram lifts the arm, easy to operate.
My wife runs an adaptive sailing program on Boston Harbor, she's got an MS clinic/program going on right now... She's also going to represent the USA in sailing at the Paralympics in China in September -she's a paraplegic, and her teammate has advanced ALS (Lou Gehrig's disease). So I have lots of experience with Hoyers on docks -her previous teammate was a 200 pound quadraplegic. Not easy to move!