Fair enough, but many of the solutions talked about as bionic duckweed on rail seem to be progressing OK at sea - battery systems that plug in to charge for each turnaround, wind power, ammonia, methanol. The marine industry is being pretty innovative at present, and things are moving from drawing board to steel quickly, while on rail it feels like it takes a long time for innovation to get off the drawing board. And OH electrification is rather simpler for rail than for sea!
If you only allocate the carbon to the passengers it is very clear cut using that calculation - the plane is much more efficent. This isn't very fair to the ferry that is using 90%+ of its deadweight for carrying cargo. I don't have the figures to hand at present, but as far as I recall if you use a figure of 1 pax = 1 tonne of cargo which is a fairly reasonable allocation (and the one used on the wiki page referenced by 37201xoIM), then the ship is generally much more efficient if the load factor is reasonable.
Sea freight on average being 100 times as efficient as air freight sounds about right, if anything I would expect it to be even greater. Those figures show sea freight as being roughly 3 times as efficent as rail freight, which seems about right too. A ferry is going to be on the low end of those numbers, as a much larger % of a ferry is wasted space than on a container or bulk ship.