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Good question. Not sure what their criteria is for "navigable". Here is a shot of what I think is one of the larger boats to use the harbor. Heading out for a whale watching excursion.

dbws-1.jpg

Maybe boats with significant hulls?

Yeah,and probably how much water the boat needs not to run aground.

I did read they have to dredge the harbor of silt every so often to keep it operating.

I think you are correct:)