What do you put in the bed of a golf cart to transport water to water plants?

I've seen water tanks that you set in the bed of a golf/utility cart and fill with water then take around to water all of your plants/trees from your golf cart. There is a hose coming from the bottom, usually they are gravity fed but some use a small gas pump. They aren't huge, but they are big enough to water some trees and plants. They are usually made of some sort of plastic I believe. I've been trying to get one for my utility cart but I have no clue what they are called. My cart has about a 4 foot bed, maybe 5 with the tailgate down. What are those things called you would put in the bed??