A voltage regulator may be cheaper and lighter
than a separate battery, but at the risk of
introducing electrical interference into your sonar.
Even with a separate battery, some people have
interference from other devices.
You are basically hooking your sonar to a battery charger
that is going to be on anytime your main motor is
running. This can't be good to get the best performance out of
your sonar.
However, the sonar should work great when your main
motor is completely turned off.
A quick test is to turn on your sonar (with main motor off).
While looking at the sonar screen, start up your main motor.
In neutral, rev your main motor up and down, and watch your sonar screen.
If you see any static, snow, lines, dots that weren't there with the main motor
off, then you have problem.