The Energy Star site will be your safest bet for information on how to get the tax credits. There are many kinds of hot water heaters, and Energy Star is very specific on what is and is not credit-eligible. For instance, it must have a thermal efficiency of at least 90%.

Don’t rely solely on contractors who may not know the details or who promise their products will get the credit in order to make a sale.

As of January 2012, the feds haven’t extended the energy tax credits for water heaters beyond 2011.