From Coastal Wiki
Definition of Sea:
The open coastal waters located seawards of the shoreline. The seawater is saline. This definition is identical with the definition of the sea in most nautical maps. The sea extends into major bays, but not into channels, creeks, rivers, estuaries and lagoons. These internal waters are characterised by having brackish to fresh water and are included in the category Coastal habitats and ecosystems in transitional waters.
This is the common definition for Sea, other definitions can be discussed in the article