I’m pretty sure it is in the manual that comes with the motors.

"NOTICE: Wire Extension Length refers to the distance from the batteries to the trolling motor leads."

 

Remember, that is in addition to the cable that comes on your trolling motor, so if you have 5 foot stock cables, and you could extend that another 5 feet and still be OK per their chart at 8 awg. Not to mention that there is likely a fair amount of safety built into that recommendation. Most of their trolling motors only have 8AWG leads on them, including my Ultrex Quest that supposedly draws up to 60 amps.

 

If it was mine (but it isn't so do as you please), I would run 6 AWG from the back of your PWC up close to where your Terrova is mounted. Can I assume your cables will be below deck? If so, run the 6 AWG to your breaker near your battery, and continue with 6 AWG to a on/off switch like the Blue Sea Mini somewhere within reach of where you fish from, and then come through the bulkhead near the trolling motor with a plug like the MK-26 or MK-28, and then connect to the trolling motor leads. I would have no problem cutting the leads so there's just enough to get to the bulkhead plug so your 8 AWG section is only as long as it needs to be. 

 

Use the Blue Sea Mini to cutoff power to your TM while charging and while the unit is sitting. I don't care to repeatedly connect and disconnect any TM plug other than when needed, and you can subtract the length of wire you cut from the stock leads and add that to whatever is needed for your 6 AWG length of cable.

 

I would not use a breaker for turning the power on and off regularly as that is not what that is designed for. 

 

I would also suggest using a quality tinned copper marine wire, which you can get online. I bought a hydraulic lug crimping tool from Amazon (Hobo Freight also has them for an almost as reasonable price) and use it for all of my ring terminals and connectors (along with the self-gluing shrink tube) on my custom cables.

 

JMHO, and good luck with your project.