I want to separate my power source for my trolling motor and my Helix to reduce interference. What is the best way to power the sonar? I suspect it won't take much battery. I just have a 14' boat with one battery currently.

A small 12v battery - 9+ AH rating - will power that Helix for a full-day's fishing. They're small, fairly light, and relatively inexpensive.

 

This 10ah unit goes for under $30.

https://www.amazon.com/12v-10Ah-SLA-Rechargeable-Battery/dp/B004WENYYE

The correct answer needs to be based on what generation of Helix 10 that you have.  The original "Gen 1" Helix 10 that I own draws 1.2 amps.  The latest generation Helix 10 is rated at drawing a whopping 2.4 amps. You also have to decide what your maximum time on water is. If "all day" to you means 8 hours, you would preferably want a battery that would power your unit for those 8 hours, while not discharging the battery below around 25-30% (for battery longevity) - you would want a battery rated between about 15 to 25 (or so) amp hours (depending on what generation Helix you have and how deep you want to discharge the battery).  If your time on water is less than 8 hours, then you can down size the battery.

 

Once you determine the current draw of your Helix (check the specifications section of the owners manual), and your maximum expected time on water, you can calculate your required battery size keeping in mind that it's not a good thing to discharge the battery too low so allow some reserve capacity...

I bought the cheapest lawn mower battery Walmart sells for around 30$ last year for my fish finder and it works great.