This location is only accessible during certain plot points
The White SeeD ship appears in an inlet in the Centra region on disc 3. You can come here any time, but you won't be able to accomplish anything plot-wise without a letter from Edea. Once you have it, speak to the leader to learn your next destination: Esthar.
In the meantime, check in on Rinoa in the infirmary once again. You'll end up in Fishermans Horizon on your way to Esthar. Note that once you do this, you'll be stuck in Esthar for some time, so now is the time to complete any outstanding sidequests.