220v water heater

nope: no reason for it in this case. it DOES need a safety ground however.
think of it as if it were a big battery and a lightbulb. there is no reason to tap halfway into the battery.

| So I work in the electrical dept at a large home improvement store, but | I'm not and have never actually been an electrician. | I recently found out that on most 220v hot water heaters (in the US) | they don't use three wires for the connection, like they do for most | 220v things (dryers, ranges) and that the hot water heaters simply have | 2 hots, and no neutral. This baffles me. Why is this? Doesn't | everything have to have a neutral?
No, a neutral is not needed. And in this case you're better off without it.
In previous times, the neutral was the only conductor connected to ground. It was needed once the need for a ground was understood. But eventually it was also understood that having the safety ground be the same as the neutral was a bad idea. So now we have a separate ground-only wire. That leaves the neutral to serve only its original purpose, which is to supply half voltages.
Clothes dryers come in both electric and gas versions. The difference is in the heating source. In the electric dryer, the heat comes from heating elements connected to 240 volts. But in both cases the motor that turns the drum, and any control electronics such as timers, are powered at only 120 volts. To keep costs down, the same 120 volt parts are used even in the model that uses heating elements at 240 volts. The neutral is merely part of the circuit to get 120 volts between it and one of the hot/live/phase wires.
Ranges also have 120 volt parts. These typically include a clock and some lights. Modern ones have other electronics as well.
It certainly is possible to run these appliances purely on 240 volts. They do that in other places in the world, like Europe, Australia, and most of Asia (220 to 240 volts depending on country). Switching power supplies as found in computers, electronic fluorescent light ballasts, and many media and entertainment devices, and readily run on a wide range of voltages, typically from below 100 to above 240 (in some cases they might need a switch setting changed to adjust some components). Many modern appliances that do connect to 240 volts in the US may even be using that voltage to power electronics, now.
Other appliances, such as air conditioners and water heaters, typically have not needed 120 volts for anything. Thus the neutral was not needed and only a safety ground wire needs to be added to the existing 2 wires. And these appliances probably never will use 120 volts directly when they are sized for 240 volts because the typical existing circuits supplying that voltage for them does not include any 120 volt neutral. So they will typically be manufactured with every part needing power using it at 240 volts.
A 240 volt circuit with a neutral is called a multi-wire circuit (in this case it is 3-wire). The grounding wire (the green one) is not counted for the purpose of describing the type of circuit. Three phase power has an equivalent, but there are 4 wires, and the voltage between hot/live/phase wires is only 208 volts (unless it is a high voltage circuit).
Multi-wire circuits have a hazard that two-wire circuits do not have. If one of the wires is broken, there is still power on the circuit. And that can cause a serious problem, especially if the different wires of the circuit are significantly unbalanced in usage. So I consider appliances that don't need a neutral, and circuits that don't have them, to be safer than those that do. A 2-wire circuit is the safest. And a 3-wire circuit is as safe as you can get in three phase as long as those wires are connected to a wye-configuration transformer (but does have some hazard that cannot be avoided in three phase, which is called single phasing).
The water heater is one of the simplest appliances, electrically, you might have in the store. It has a thermostat, contacts to open and close the circuit, and heating elements. Supplying it with 2-wire 240 volt power (with a ground wire, of course) makes it one of the safest appliances around. Very small water heaters might run on 120 volts for special purposes like "point of use" (they don't need all the power of 240 volts, and this saves having to install a special circuit in many locations).