Skimmer
Over water, over lava, and — once trained — under the water as well.
The skimmer is bought from a mount trader in Path of Fire after the story reaches the Elon Riverlands. Like the springer it costs coin rather than a collection.
Complete the Skimmer Ranch task region in Elon Riverlands to unlock the Skimmer Mastery Track.
The skimmer is the mount whose later masteries change it most: what starts as a way across water ends up as the fastest way through it. Note that its deep-water training sits in a separate mastery region from the mount itself — the game groups it with Secrets of the Obscure content, so those points come from elsewhere.