As I was leaving the boat around 1630, having completed my chores for the day, I gathered my gear and prepared to get off onto the dock. I stepped out the shrink wrap door, I put my right foot on a boarding step, and my knee promptly gave way. My left leg was still on deck but was tangled up in the shrink wrap zipper that was inside the door. I fell backwards with my head in the water between the boat and the dock. Try as I might, I could not pull myself back up onto the boat. My left foot was at deck level. The water was cold!!