If I'm not terribly mistaken it should suffice to just add the ability to the export_descr_unit file in the data folder.

Backup the file.

Open the file.
Search for "chariot" (the celtic ones are the first in the list), go to "attributes" and just add "sea_faring," as first entry (before "frighten_foot")
Save.
Delete the map.rwm (not sure if it is needed, it can't hurt either).
Done.