Neither the homebuilder nor the homebuyer would qualify for a tax credit for an energy efficient non-solar* water heater installed in a new ENERGY STAR labeled home. IRS rules require that the property be installed in a dwelling unit that "...is owned and used by the taxpayer as the taxpayer's principal residence" at the time of installation. So, unless the homebuyer owns the home and has been using it as his/her principal residence at the time the water heater was installed, they cannot claim the tax credit for the water heater. There is no builder tax credit for an energy efficient water heater.
*Solar water heaters are covered at 30% of the cost.