I am rewiring my house/start battery right now because of this. As wired by installer everything is either on battery 1, battery two, or both.
On occasion, when I start the motor, it'll pull the voltage down far enough to cause my Lowrance HDS unit to reboot.
The boat is 12 years old. Only became an issue for me as second owner because I put more sophisticated electronics on board. First owner never perceived a problem.