A tugboat (also called Seatub was a large boat used prior to the Great War.
In the Capital Wasteland, they appear frequently in the Tidal Basin, Potomac River, and some small lakes. They can also be found throughout the harbors of Point Lookout. Many have loot inside them.
In the Mojave, they only appear infrequently in Lake Mead, and never house loot.
Tugboats appear in Fallout 3 and Fallout: New Vegas.