Except the Navy calls sub tenders "sub tenders" and not "offshore support vessels". And the only two they have are in Guam and Diego Garcia.
It was probably one of these - Link
My brother in law lives right on Hood Canal, just a couple miles from Bangor base... we've seen many times the subs coming and going, and they're usually flanked by a couple ships that look like barges. I always assumed they were mainly to hide the subs as they make their way out of the area.
I do know that getting anywhere near that base is bad news... one night my brother-in-law's boat broke free from it's anchor overnight. My brother-in-law had left for work early the next morning, and my sister-in-law discovered that the boat was drifting toward the base, so she got in the Zodiac and went to get it. She was met by a couple of boats with well-armed Marines who were very serious. They eventually did tow their boat back to her though.
Thanks LD