Ocean water is sometimes used when close enough for helicopter dip tanks or scooping airplanes. However it is rarely used.

Ocean water is salt water. While during a fire that may seem unimportant, it’s really very important.

First, salt water can damage equipment not intended for salt water use. Salt water increases corrosion severely.

Secondly, large amounts of salt water on small areas as dropping from an aircraft will leave the salt in the soil. This will alter the soil chemistry and many native species will not be able to grow or may die. This will cause barren areas where watershed is lost and erosion will destroy the topsoil, as well as now spreading the salt downhill causing more damage.

Lastly the logistics of moving enough water more than a very short distance inland would severely complicate an already complicated operation. Most big fire campaigns are managed like armies in coordination of personnel and equipment.

An example, the city of San Francisco in California has a program to pump water from the bay should another major earthquake damage the water infrastructure in the city. They have a “portable water main” system with larger diameter hose and fittings, and pumps to lift and relay the water along.

With several trucks dedicated to this mission the best they can do is one to two miles inland. And only a couple of thousand gallons per minute, which when your city was leveled by an earthquake is better than none, but not enough for a major firefight.