Preventing electrical back flow on AC circuit breakers
Can't be done. 

You should get a new meter installed for the water heater. 

It's up to you how you get the tenants to pay for the usage. However without also adding hot-water meters for each tenant there is no way to know who is drawing the water and using the power.

Normally landlords embed the common costs in the rent for all units. Or swallow it as cost of doing business.