This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
The White SeeD ship is hidden in a random inlet somewhere in the portion
of Centra northwest of Edea's house. Because its location is randomly
selected between several inlets, it could be anywhere.
Sorry this isn't more helpful, but its location is going to be a little
different for everyone... Don't give up hope, though; it is out there.