Who conducted maritime surveillance before Coastal Command

Were any units conducting maritime surveillance between 1918 and the formation of Coastal Command in 1936?

Yes, but only a small number of coastal-area squadrons, while much naval reconnaissance still depended on shipborne Fleet Air Arm aircraft.

A few RAF squadrons conducted maritime work between 1918 and 1936, but their numbers were too small to provide a centralized global reconnaissance system. Coastal Area theoretically covered long-range patrol, while the Royal Navy relied heavily on Fleet Air Arm aircraft carried by ships for local reconnaissance. The existence of patrol units did not provide the scale and coordination needed for a mature maritime-surveillance force.