Get your taxes done using TurboTax

In a rental a water heater is like a stove for depreciation ... they are both 5 year property but if you want you can call it an improvement to the property to get the 27.5 option.  Or  opt out of the bonus depreciation to spread it over 5 years. 

Not sure where you are getting lost ... if you really want to have it depreciate longer than 5 years then call it real estate instead of an appliance.