The folks at the Edina pond have a small fleet of what they call "Polo" tugs. At some point in the clubs history, there was a one design tug that many people built to play a version of water polo.
As any good group of good natured modeling enthusiasts would, the one design didn't last long, different scales were used, etc...but the boats are still around and tend to be great at pushing other boats around. I believe George Pfeifer
's is a variant of this design.
With some padding on the front these tugs do a good job pushing other boats around.
For sail boats like Fairwinds, Seawinds, etc, a rope with a tennis ball works well too. Do a couple circles around the stalled sail boat and tug it in to shore.