We were recently out on the water and stopped to swim, shutting off both motors. When we went to leave only the port motor would start. We tried to jump the starboard motor with a jump back and had no success so we called boat us and they tried to jump it and still nothing so we got towed in. We replaced all 4 batteries on the boat and are still getting low volts around 11 when at the dock on shore power. Even our VHF radio alarms low battery with the new batteries. Any ideas why we are getting such low voltage?