Whirlpool water heaters are made by AOSmith company, which owns American, Reliance, State, Craftmaster and makes water heaters for Kenmore and Whirlpool.
These are good heaters.
Register Whirlpool product at following link:http://www.whirlpoolwaterheaters.com/support/warrantyreg.aspx
It is not necessary to register product for warranty to be valid.
And if you move to new home and take water heater, then warranty does not follow. So it might be disadvantageous to register product in that circumstance.
You can see brand, model number, and serial number located on label on side of tank.
The serial number is unique and is key for warranty coverage, even if you lost receipt. Serial number shows year of manufacture in first two digits. http://waterheatertimer.org/pdf/Find-Age-of-water-heater.pdf
It is not necessary to register water heater for warranty to be valid, as long as following applies: Original owner, single-family non-commercial residential installation, and installation that conforms to instructions shown in product manual, and water heater is maintained as shown in product manual.
If you purchased extended warranty, then remember the warranty is carried by a third party that also requires same as above.
And the warranty may be pro-rated, rendering it nearly valueless in later years.
Typical water heater warranty:
First year of warranty covers tank-parts-labor.
After first year, the warranty shrinks to tank-parts.
The math shows that shorter warranty tank has greater percentage of full coverage.
Convert ordinary 6 year tank into 9-12-15 year tank by maintaining water heater. Especially cleaning air intake located on bottom of tank, inspecting anode rod to prevent rusting, and following maintenance schedule shown in product manual.http://waterheatertimer.org/How-to-maintain-water-heater.html