The calculator is currently being revised to include ferry travel.
Estimates of CO2 emissions are:
• 0.20 kg / mile  (apparently referring to a foot passenger) - since a 50 mile ferry journey is said to generate 10 kg CO2 (Ullapool to Stornoway, taking 2hrs 45 mins)
• 0.75 kg / mile [ML, 6] (apparently referring to long-distance ferries, travelling with a car)
• 1.03 kg / mile, calculated from data in source , which found Mersey ferries generate 48 kg CO2 per ferry km, and the average ferry occupancy was 74.7 passengers.
• travelling with a vehicle must be associated with a greater CO2 emission than as a foot passenger, and so must a journey requiring an overnight cabin - and then the extra CO2 must depend on the number of people sharing the car / cabin
• travelling on a cruise ship with swimming pools and other entertainments must use more carbon than a simple ferry
• faster ferries e.g. very fast catamarans must worsen the emissions per passenger mile
See Reference List