Originally Posted by Benz
Looks like an oil
barge. When the tug fits into a scoop at the back, it's called a "Composite Barge," since the tug and barge operate as one unit in that configuration.
In the trade
they're called an ATB - Articulated Tug/Barge. I think the nav rules use the term composite.
The tug has hydraulic rams that fit into notches in the slot.
I can't tell 100%, but I'd also guess liquid fuel
(e.g.; oil) of some kind. There's what might be a set of pipes running along the center, and what might be a manifold on each side midships where all the pipes terminate to attach to a matching manifold on the dock
Plus, there's a lot of that type of traffic in Buzzards, so it makes sense you'd see some in Google images