The Mars used modified fuel cells to carry 7,000 gallons of retardant. To load the water, it would "skim" a body of water with it's scoop extended and as the water was gathered, it would mix with the phoscheck.
Here in Northern California, there used to be WWII aircraft converted to fire-bombers:
B-17s, TBFs and most recently (now retired), the PV Neptune.