An Amphibious unit may enter underwater provinces without penalty. Like Aquatic units, Amphibious units have -1 Encumbrance when underwater. This is not reflected in the recruitment screen.
Poor Amphibians may also enter, but will receive penalties to their attributes: -3 attack and defense skill, +1 Encumbrance, Combat Speed halved. Some units, notably Turtle Warriors and other Mermen, will be marked as poor amphibians in their land shape but transform into full amphibians when moving underwater.
Both types can swim over rivers.
Waterbreathing is typically an externally-bestowed form of Amphibious (from items or Gift of Water Breathing, for example), and has the same penalties as Poor Amphibian underwater. Aquatic units cannot benefit from it.
See also: Aquatic.