Seeking information about submarines in Vietnam

Hello. I'm seeking information regarding the location of submarines during Vietnam as I am trying to work through the Blue Water Act. I know my father had a Vietnam Service Medal and he entered the Navy in 1967. He served 25 years, so obviously through the end of the war. The qualifying Blue Water Act "offshore" definition is very defined - how can I find out where exactly he was? Seems like there isn't much info out there regarding subs during Vietnam.