A "tug boat" is a small and powerful boat that helps larger boats move and steer in tight places, like ports or rivers. It pushes or pulls the bigger boats to help them move safely. Full definition
The crippled cruise liner Carnival Triumph limps into port guided by tug boats in Mobile, Alabama (independent.co.uk)
On my snorkeling trip with Ocean Encounters I saw some of the most beautiful fish and marine life I've ever seen, and even explored a sunken tug boat! (brooklyntraveladdict.com)
Jan Tiura, the first woman tug boat captain in the bay area, had been interested in building a modern cabin for decades to set on her property in -LSB-...] (tumbleweedhouses.com)