Hello!
First of all, I will admit, even though I am not an OKW'er, I did vote, just to see the results. You can count 1 less on the choice three, if you like.
I however, did vote honestly. After visiting over there, just to see the resort for ourselves, We did take the ferry to DD. My personal opinion would be you need more than one. The time between runs is huge. Changing the size of the vehicle won't make it get there and back any quicker.
Edit: I also seem to remember the boat driver mentioning that part of the run was a no wake zone, so, if I am recalling that properly, a bigger boat would have to go even slower, hence the runs would be spaced even further apart.