I had the same question last year and couldn't find a good source of information. What I finally did was build a little spreadsheet with a column for the date and port name for each stop on the cruise. Then I went to the Web sites for each of the major cruise lines, found cruises to the same general location and looked ontheir itineraries for common ports. Anytime I found a match I added in the appropriate column of my spreadsheet. When I was all finished I had a list of each ship that was in the same port as us on a given day. It was a fair amount of work but turned out reasonably well.