I would run two 4 gauge power and ground with marine battery cable. Another important suggestion is to have an on board battery charger and keep boat plugged in when not in use.
What I often see is that people leave the dock with a 50% charged battery since they dont use the boat every day. They run only 15 minutes and from there it does not take much to run the batteries down. Bear in mind the lower the voltage the more the amps will pull so they run the battery down even faster.
If you let your batteries go dead a few times, they will never recover fully. It is important to always keep the batteries stored fully charged.