Dante,what outboard do you have?
Most manufactures have a charging kit than can be installed as an aftermarket accessory.
It essentially will connect your battery to your outboard alternator and when you motor is running, it will charge the battery.
If you're handy you can install them yourself or you can go through a dealer.
You will then connect your sounder to the battery as per normal.
I have operated my current and previous boat using this method of charging and I have never had to charge a battery through a charger at home again.
I would highly recommend that you look into this option as it is far more convenient than hooking up a charger (easy to forgot to do as well and then have no battery power the next time you use your boat, I know from experience ).
It is probably comparable in price as well.