First you need to edit adj-defs.csv (in map folder - I take if for granted that you patched your game at least to 1.08). The format is rather selfexplaining - just copy an existing strait and change the 3 numbers to appropriate ids for Kent, Calais and whatever the seazone is inbetween.
straits.csv is actually optional - it defines the pixel values of the 'islands'-graphic which is showed on the map. It has no other purpose (the actual strait will work already if you followed the first part). Personally I'm too lazy to bother about finding out the exact pixel values needed here, but if you're a perfectionist you can edit that file appropriately (again the format should be obvious - these are x,y coordinates counting from NW).