As I was daydreaming, I suddenly noticed the depth sounder was reading 5 meters!  Immediately I put the engine into neutral and we slowed, as we slowed the depth reduced, I gave a kick astern and we stopped.  The depth now read zero, that means nothing under the keel!  I checked everything, a flashlight over the side, we should be in deep water.  Then I realised my mistake.  I was so tired and my brain so befuddled, I had been reading the wind instrument instead of the depth sounder.  As we reduced speed so the relative wind reduced to zero as we stopped.  Clearly, I had not been thinking straight.  I was very tired.