Photos can sometimes cause a distorted perception: But if you removed the exhaust on that side of the engine, could you get the water heater in without disassembly?
When you say exhaust do you mean that big rectangular black hunk of metal behind the oil filter? I dont think that would do it. I think the entire remote oil assembly would need to move.
Anyway, the isotherm units look promising. The SPA 20 or slim 20 would work great. Being cylindrical without the square box is a huge clearance advantage in my particular engine room. I could go larger with the slim series, but we dont use it that much away from dock. I just dont want to start deferring items.