Water Heater on/off switch
No, because you are talking about a 30A load, and that is way, way, way too much current for common switches.  By the time you pay for the exotic switches you would need, you would just be better off wiring a subpanel.  

It might be possible to wire that legally, but I rather doubt that's what happened.  It sounds like an ugly hack job that's just waiting to electrocute someone.  I wouldn't touch any of it with a 10-foot pole, especially if you are a tenant in somebody else's house. 

You simply need to train your laundry people that they cannot run the dryer 30 minutes after anyone uses hot water. That shouldn't be in the way of washing clothes; hot water is only used at the beginning of the cycle, and the water heater should have caught up by the time the cycle ends and it's time to go in the dryer.  

Stacking loads (drying one while the next one is in the wash) is not allowed unless they select "Cold" water.