Against a Sunfish Edit
Make your way to the surface and boost into the sky. You'll move quicker. As a Seagull or Pelican, stick to the top of the sky.
As a Sunfish Edit
Feed on players with low health or animals that are lower than you. They are easy targets and you will not jump away after hitting them. The Sunfish does have relatively low health, though, so avoid other top tier animals.