the count lands from sea. Can he then play double back to return to the same sea area from which he just debarked?

I would say you can, although I can't think of any reason you would want to.  Firstly you would have to pay 1 blood for re-embarking and, secondly,  in many cases you will have very limited movement options once you've done this since you can't go to any locations that are on your trail at this point.

I guess if you realize that you've made a mistake by disembarking you could use this strategy...

thanks for the reply. This was one of those "unique" instances. .The count only legal move after debarking was to a location with two hunters. He would have to fight  them as a group, in daylight, with the hunters having the Sister Agatha ally card face up and the count with only four blood remaining. Not a good prospect for the count. That is why he tried to backtrack to sea.

Yes, the Count can double back from land to sea. But power cards (like double back, feed, etc.) can't be played while the Count is on sea, so it can't work the opposite way around (double backing from sea to the port he just left).