Dave from Beckman Electric in Orange County CA says you have a open neutral conductor between the utility transformer and the main panel of the house causing the swimming pool to be the path of least resistance for all 120 volt electrical loads in your home. Have an electrician check the main panel and make sure there is a wire connecting the neutral conductor from the utility company to the neutral buss in your panel. If not the neutral is being completed through the grounded metal can of the service and the service is grounded to you pool causing voltage in the pool. If the electrician cannot find anything then do your multi meter check again testing the voltage as you turn on 120 volt lights and appliances in your home. If the neutral conductor is open you will notice the voltage rise as more 120 volt devices are turned on. You may have to get the utility company to check the connection of the neutral conductor at the transformer of other point of connection.