Originally it was connected above the hotter heater (Starboard side for the 77's). But it didn't always have a shutoff, and many have been re-plumbed over the years.
The line runs from the hot water to the ceiling of the master bathroom, then makes a right turn and go's to the aft sink.
I moved my shutoff to the master sink. Mostly because it was cracked and I could not fit or pull a new line above the showers to the engine room.