I run two of the 6 volt 4 amp-hour batteries in my Springer, connected in parallel. They are on platforms at the bottom of the curve of the hull, and are held by Velcro strips on the bottoms and on the sides against the hull. The batteries lay on their narrow sides. I can launch the boat with a "Springer launch," a.k.a. throwing it, and play rough and tumble water polo, and the batteries have never moved. I run all day on a charge and seldom come home with less than 6.1 volts left.
Add a 3/16 inch wide "V" to the trailing edge of the rudder to improve the turning radius. Get the prop shaft as near level as possible to minimize the boat's tendency to dive at full speed.