If you are a personal homeowner (not a landlord) then purchase of a new hot water heater is not a tax deduction. It might be eligible for an energy efficiency credit of up to $600 (for a conventional hot water heater) or up to $2000 (for a heat pump hot water heater) if it meets the required energy efficiency standard as shown in the instructions. (You may have to contact the manufacturer to find out if it meets the required efficiency standard.)
https://www.irs.gov/forms-pubs/about-form-5695
You claim the credit when the unit is placed in service (installed), regardless of when you pay for it. Because it is a home improvement, the net cost (after any rebates and credits) increases the adjusted cost basis of your home and may reduce your capital gains when you sell.
There is no reason to not claim the credit, if you are eligible, even if you plan to sell your house soon.
If you are a landlord,
You place the property in service and begin depreciating it when it is placed in service. When you sell, you must recapture any depreciation you take or could have taken, so you should list it as an asset even if you plan to sell the house soon.
