A tugboat is a small sturdy and powerful vessel designed to push or tow other vessels and barges
Tenacious
You will see them in every sizable port; smart, businesslike small ships, low in the water and surging out to a large inbound ship. Tugs represent power for pushing and pulling, an engine with just enough hull for adequate buoyancy. Thick fenders for close-quarters work, pushing a big ship alongside the quay against the wind, hauling her off at the end of a towing wire.
The United States Coast Guard has donated a number of vessels as artificial reefs over the years. They seem to be sensitive about having their vessels shown in a "bad" way, and typically paint out all the markings before turning them over.