Workerdone,
I agree that an immersion heater wouldnt work for any kind situation where you are trying to warm a flow of water like what OnBeechRoad is doing.
However, if you are letting water warm up in the sun over a couple of hours, it should add some heat. Would it actually be worth it like you said? Meh, maybe not. Might be worth trying though for a few bucks.
Another idea that I had would be to use a generic stick on engine block heater. Amazon sells a 150 watt 110v "Universal Hot Pad Heater" that cost less than $35. Then again, you are looking at hauling some sort of metal bucket (to distribute the heat and not melt), and a reasonably large power source for the 110v, so generator, running car, or large batteries. At that point, you might as well just get a propane tankless water heater for like $120 (water pump not included) and be done with it, but again, you are hauling around a lot of extra gear that can take up a fair amount of room.
The more I consider other solutions, the better OnBeechRoad's solution looks!