Underwater provinces, often abbreviated UW, consist of one of the principle terrain types for Dominions 5. Consisting of Sea and Deep Sea terrains, these are by far the most unique terrain types, due to many related mechanics which set them apart from other terrains.
While many terrains hinder movement to some extent, it is not possible to occupy an underwater province at all without any of the traits: Poor Amphibian, Amphibious, Aquatic, or Water Breathing. The only exception to this rule is Assassination battles triggered in the Magic Phase (known because of old Infernal Disease mechanics, no longer relevant).
This sharp divide between which units can and cannot go beneath the waves is likewise reflected in the nations of Dominions 5. Nations whose capitals start UW are considered Underwater Nations, and their national rosters are taken up partially or entirely by Amphibious, Poor Amphibian, or Aquatic units.
Some items convey the ability for otherwise land-dwelling units to travel underwater. Usually, these items provide Water Breathing or Gift of Water Breathing. Similarly, Aquatic units can only travel on land if they have an item which conveys Air Breathing, although troops cannot be brought on land this way (in an un-modded setting) because no item conveys Gift of Air Breathing. Land units (without the ambhibian tag) fighting underwater get the same penalties as poor amphibians (-3 Att, -3 Def, halved CS).
Over-water terrain types are also sometimes modified when coinciding with an underwater terrain. For example, Forests become Kelp Forests and Mountains become Gorges.