Marvellous. At last I understand. When I'm at sea my boat is floating in the water that is there, but in a lock it is kept afloat by the water that isn't there. Simple.
Fisherman, that's great and the same as my reaction to the idea that mystical water that might be elsewhere, feels frustrated at being displaced and exercises force at a distance by quatum tunnelling to keep the boat afloat in the dock.
My question was not a joke, I was really baffled by the issue and I have now read an enormous number of answers to it. Discarding the those that ignored the question I don't think I am any closer to a clear answer. I just cannot believe that a big ship can float in a tight dock in just a cupfull of water. But I repeat that I took two slighlty different sized porridge bowls (from two passing bears) and found that I could float the smaller one weighing 12 oz in the larger one using only 4 oz of water.
A tip. put two plastic bowls, one floating in the other, in the freezer. add a few interesting rose petals etc to the water first. Remove bowls from ice and you have a lovely ice bowl to serve a cool summer dessert in. And it won't need washing up.