If you have a gas-fired water heater, check to make sure it is venting properly. Light a match next to the vent and wave it out (don't blow it out). See if the smoke is pulled up into the vent. If it isn't, have a professional inspect and repair it. Otherwise, carbon monoxide and other combustibles can build up in the home.
Check around the base of your water heater for evidence of leaks. If your water heater is over 5 years old, it should be checked monthly for any leakage or rusting at the bottom. If water leakage or rust is found, the water heater should be replaced.