A sea is a province which is underwater. Population types and magic sites are different in seas than they are on land.
Most units cannot enter or survive in seas without magical help. On the other hand, units with the Aquatic ability are native to the sea and cannot leave it without magical help. Units with the Amphibian ability can move freely between land and sea. Units with the Poor Amphibian ability can also do so, but suffer penalties when fighting underwater. Certain spells and magic items can change these rules.
Many spells cannot be cast underwater; a few can only be cast underwater.
Commanders with the Navigation ability can sail over seas, traveling from one coastal province to another that borders the same sea and taking all troops under their command with them.
Seas are immune to some special and magical effects, including the Heat and Cold scale. Many nations cannot build fortresses underwater.
A few nations have their capitol in a sea and most or all of their national troops are aquatic or amphibious.