The belt of coastal waters extending up to 12 nautical miles from the baseline over which a coastal state has full sovereignty.
In practice
For a master, entry into territorial waters triggers important obligations. MARPOL special area rules, domestic environmental law, pilotage requirements, and customs reporting obligations all typically apply from the outer limit of the territorial sea. The vessel's AIS must be active; certain flag states also require advance notification of entry. Any pollution incident within territorial waters is subject to both flag state and coastal state jurisdiction.
Regulatory detail & full definition
The territorial sea is the belt of coastal waters over which a state exercises full sovereignty, extending up to 12 nautical miles from the baselines defined by UNCLOS. Within territorial waters, the coastal state may apply its full domestic law — including criminal, civil, and regulatory law — to vessels in passage, with one significant qualification: foreign ships enjoy the right of innocent passage, which the coastal state must not hamper.
For a master, entry into territorial waters triggers important obligations. MARPOL special area rules, domestic environmental law, pilotage requirements, and customs reporting obligations all typically apply from the outer limit of the territorial sea. The vessel's AIS must be active; certain flag states also require advance notification of entry. Any pollution incident within territorial waters is subject to both flag state and coastal state jurisdiction.
Innocent passage requires that the ship's transit be continuous and expeditious, that it not engage in activities prejudicial to coastal state peace or security, and that it comply with coastal state laws on safety of navigation and protection of the marine environment. Submarines must surface and show their flag. Warships enjoy a different regime. For most merchant ships, the key practical point is that territorial waters mark the zone in which coastal state law enforcement powers are fully effective.