Ok here people; the guy's original question was what do I need to make what I have work for a day of fishing. Some here ae trying to outfit him for a tournament. LOL Doing what you asked is very simple. Get yourself a couple of group 24 or 27 deep cycle batteries. Maybe one that is classed as a start/deep cycle (Battery Bull? I don't know). Hook both batteries to a combination switch.
This will allow you to separate the batteries from each other or connect them together, if you run the starting motor low with your electronics (fish/depth or whatever finder) low. you'll have the trolling motor battery in reserve or vice a versa?