Put 'em together and you get P=E²/R
So power (watts) go up when voltage goes up (exponentially), and goes down when resistance goes up.
So, for a given voltage, a high-watt device shows a lower resistance (or impedance) in the circuit than a low-watt device.
Clear as mud?
As for
why a water heater has a lower resistance and thus draws more power than a light bulb... because it's designed to. It takes more energy to heat 40 gallons of water than to light up a room. One could make a 100 watt water heater but the water would take a really long time to heat. You could stick a 1000 watt bulb in your ceiling fixture but you'd go blind, melt the fixture and burn your house down all in one easy step.