Well, as far as I know you should just stay in the area and sink some more. That's what I do and the objective always get completed. I don't know if you fail the objective, but I think the "sink enemy merchant shipping" part is secondary to the first "deploy to" part. If so, you should be good. I'm sure someone else can give a more conclusive answer.
On Fremantle;
http://home.st.net.au/~dunn/usnavy/f...marinebase.htm
Google, and yes even Wiki, are your friend. Lots of info on individual subs and, well, anything really.