Spent hours searching the www, still can't find the DCH-6 Float Bomber Capacity
If you're really stuck for the answer, I can ask when I see the maintenance guys next spring. I knew the gentlemen who designed the mod, though they're both pretty well retired now.
In the mean time, this information should get you close.
https://www.bushplane.com/index.php?id=waterbomb
There is a minimum amount of water to be carried before it actually reaches the ground in a salvo. I was doing some design work to have a Cessna Caravan become a water bomber, but it won't carry enough weight in water to make it effective. The Twin Otter seems to be about the minimum, but they work well....
I have visited this fine museum a number of times, and highly recommend it
Also, have a look for the book: Aerial Firefighting, by Wolfgang Jendsch. I have a copy, and it's a very informative read.