The Amphibian is a unit exclusive to the Aquarion tribe. However, they can be mind-bent to another tribe using Mind Benders and still have the same characteristics as an Amphibian but with a different color scheme. They are the Aquarion version of the Rider. They have 2 movement when in water but 1 movement on land. This also applies when escaping.
The Amphibian can swim in shallow water automatically since that is unlocked with the same tech as Amphibians, but needs the Free Diving or Navigation technology to be able to enter ocean.