They are often used as background material, set dressing, or simply extras and have no speaking roles, nor faces.
One notable appearance of some of them was at the beginning of the final episode Bigg Freeze, where two steam-driven fishing boats pass by. Another boat of a similar appearance was in the episode Up River, also towards the beginning.
Some of the boats were recycled after TUGS had ended. Those recycled would later on be used in their sister series Thomas the Tank Engine and Friends, from which they can be seen in some episodes. Up until the CGI-animation series went up and running, it is currently unknown where the models are currently are.