Your dead phase is backfeeding via your 240V appliances.
If your 240V appliances had 0 resistance, your dead-phase side would light up at full voltage.
As things are, the 240V loads are in series with the 120V loads on the dead phase. That is causing the voltage to settle out where it is. You can manipulate that by turning on 240V resistive loads or turning off 120V loads. (I wouldn't run a motor load under these circumstances.)
Keep in mind that your water heater is going to work anyway, so it will be cycling on and off. That is to say, when it cycles on, it will parallel itself with the other 240V loads if any. It will use its share of the power to warm the water. Most water heaters are well enough insulated that the water heater will make progress even at 1/16 normal heat output (60V instead of 240V), and eventualy it will reach target temperature and shut off. You can't do anything about this, except waste hot water.
There's an earlier edit where I talk about phantom voltage. It's wrong here.