Whales leave footprints on the water from the motion of their tail under the water. You can see what this footprint looks like by the photo on the left, and it makes it easier to track where the whales are. They can even leave these footprints when they haven’t surfaced above the water, but most of the time you can see the footprints after the whale has pushed it’s tail on a upward stroke.
The footprints are also in the shape of a perfect smeared circle on the surface, which is created by the vortex of the whales tail as you can see from the photo as well. You can see them when the whale stays close to the surface of the water as it moves along. The footprints can also be seen in a line from the direction the whale is travelling, before they slowly disappear.