Minecraft wiki entry for Elevators, associated text: Making the elevator gates taller. A 3-block gap in the wall is easier than a 2-block gap to get through.
Replace walls with dyed glass panes on each floor. Glass panes block flowing water, but the gap left by each diagonal pair is just enough for people to walk through. In essence, the entire floor is the gate. (Undyed glass panes arranged in a column is invisible!)