15th Transportation Battalion (Terminal)
(Source: David Olson)
I believe this to be a J-boat. Notice the sign "Property of 81st Boat Co, APO 78". The 81st Trans Boat Co (Composite) was alos located at Rochefort and operated several different types of water craft (LCU's, LCM-8's, LCM-6's, T-boats, J-boats) in support of lighterage operations during NODEX exercises as well as performing routine short-haul operations along the western coast of France.