Here's a list of the custom submarine designs made by fans of Sub Commander that I currently know about. If you've made your own submarine in the game, feel free to post about it here too!
December Class
Created by Zombie_2371 from the Facepunch forums.
Download link:
http://puu.sh/bMDhF/40368fb8d5.rar
Pinto Class
Created by EuchreJack from the Bay12 forums.
Download link:
http://s000.tinyupload.com/?file_id=...70232074369061