Single.
The Boat US tow boats are ALL single engine, and they come out to rescue the broken down boats.
Speed and fuel efficiency are closely linked by weight: the less weight the faster you go, and the less fuel you burn. A twin, unless absolutely wide open, has no performance advantage. But at all speeds, including wide open, it is far less efficient. 1500 lbs for the engine and drive etc, and then add in the extra fuel weight, and you are really dragging a lot of junk. All that junk sucks gas, and slows you down.
Also, if the engines are not running at exactly the same speed, one is pushing, the other is dragging.
I really wish my boat had a single engine instead of twin big blocks.