How to treat your rental property expenses can be a little confusing sometimes. IRS publication 527 provides strict guideline about how rental property expenses are treated on your tax return. Please follow this link IRS Pub 527
In review of your questions, the new water heater and exterior staircase is considered capital improvement of the property and prolong its life and adapt it to new uses must be depreciated over a period of time rather than deducted as a current-year expense.
Attached is a screen print file from IRS Examples of Improvement for your review.
