You may find, like I did, that your electronics will kill your cranking battery in a few short hours. Today’s electronics can be juice hogs. I went through several crankers trying to figure what to do. I have a dedicated AGM battery with 2 Humminbird Solix units on. That are killing a battery grave yard dead in less than a year.
If you have a setting on your sonar unit that will show supply volts, I recommend that you set it to show so you can keep an eye on your cranking battery’s state of charge so that you can avoid being stranded.