When I was younger I tried using a 55 gallon Blue Barrel and left it sitting in the sun. I already had the barrel. The water only heated up a slight amount. I thought about painting the barrel black, but gave up on the idea since I was tent camping and didn't really feel like hauling the barrel around and it was a lot of water to heat at one time in such a large container. There were also other things available like Zodi Hot Man portable burner units.
55 gallons is a lot of water to heat at one time in a large barrel while camping. You might be better off using 6 inch black ABS pipe cut into 6 foot lengths and using elbows to connect them. You could hauling them on top of your vehicle roof rack. I've seen YouTube videos where this looks like a pretty effective way of heating water in the Sun. Especially since you're dealing with 6-inch diameter spread out over a longer area of pipe being more efficient to heat.
