Direct answer: Marine tar is tar used in maritime settings, especially pine tar or wood tar used on boats, rope, rigging, and timber exposed to water.
The sea created one of tar’s biggest historical markets. Wooden vessels, hemp rope, canvas, and natural caulking materials all needed protection from moisture, salt, and decay. Pine forests supplied tar, pitch, turpentine, and rosin to shipyards and fleets.
Marine tar may be brushed onto rope, used in traditional boat maintenance, mixed into heritage finishes, or discussed in relation to caulking and waterproofing. The exact product depends on the craft and period.
Marine tar is a use category. It usually points to pine tar or related wood tar, but the phrase itself does not define a chemical composition. Modern marine sealants can be synthetic and should not be confused with traditional tar.
In short: A use-based phrase for tar in maritime settings. Usually associated with pine tar, rope, and wood. Part of the naval stores world. Different from many modern synthetic marine sealants.