It looks like you have the drop mechanism worked out. If I could offer an idea; have a float several feet from the rescue harness and deploy as pictured (drop down until float rests on surface then move laterally while lowering until release of the harness). Now when the rope is pulled from shore the rescue harness should be pulled to the victim. This should get the harness to the victim in one try and would also work for a general water rescue. The length between float, harness, and victim are up to you but you can practice this procedure on a fixed object (piling, anchored float, etc).
EDIT: Use a floating rope between the float and rescue harness.