We call them air tankers. Water is used by amphib tankers like the CL215, but water is not a very effective way to fight fire. Fire is fought from the ground, anyway...not from the air. From the air, we simply control it by directing where it burns...using retardant for the most part.
Yes, firebombing takes place, using fire from aircraft, but that usually involves drip torching or ignition spheres to set fires as a firefighting technique (backburns, etc).
In the US Neptune, Inc, operates a fleet of P2V's as air tankers. Years ago I flew for another operator that used several, and was carded in the P2V-7. Then we hauled 2,400 gallons of retardant.
The most common airtanker in Oz is the PZL Dromader...what we call a SEAT in the US (Single Engine Air Tanker). I operated those for seven years, too. A SEAT is an effective initial attack tool on fires, being used against fires when they first start, but it's ability to be effective diminishes as the fire grows in intensity.
The DC10 and B747 platforms have some limited value...but retardant application is most effective when done in a precision format, flown by experienced crews (which neither the DC10 nor B747 have...and the B747 isn't fielded yet). Dropping large quantities of water or retardant is impressive to the public, but not necessarily what's best for tactical application in firefighting, in most cases (unless one has very long, straight firelines...which is seldom the case).
We don't put fires out from the air. Dropping water or retardant doesn't do much on a large, active fire. One generally isn't going to put it out with water. Much of the retardant application is done in advance of the flame or offset from it, and is used to drive the fire in a direction toward natural barriers, divert it around structures, etc.