I agree that the last posted time is 9:20 leaving OKW for DD. But be sure and check with the CM driving the boat.
I was there in September and decided to skip a DD bus so as to take the last boat just for the fun of it. When they pulled in a little after 9:00, they told me that they wouldn't be making the 9:20 run. It was a clear and calm night but I asked anyway if it was a weather related issue and they said that it wasn't, they just decided not to make the 9:20 run!