If the server is your own and you have access to the files you can do the following to enable the airships:
--In your FTB launcher--
Select the modpack, click "edit mod pack"
Find ArchimedesShips in the disabled list.
Select it and click Enable.
Done!
--On your server--
Save and Stop your server.
Navigate to the folder where your server is located.
Inside there should be a folder titled "mods", open it.
In this list look for "ArchimedesShips-1.6.x-1.4.5.zip.disabled"
Rename it, remove the ".disabled"
It should now say "ArchimedesShips-1.6.x-1.4.5.zip"
Done!
-------
Launch your server, start your game, and away you go!
From my experience (running a server with 5-6 people at peak hours) it is relatively safe to use. The passenger seat tends to desync between server/client with the passenger (they see their self fall out, they are still seated according to the server, strange things start happening, once ship is parked all the problems are resolved with no harm done).