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 s 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.