Doesn't have to be 2 batteries. You just need 4s voltage wise. You can use (2) 2s batteries in series or (1) 4s battery either is fine. Look for some 4500mah - 5000mah and 30c or higher. Check hobbyking.com for some Turnigy packs or hobbypartz.com. As your first boat you picked a difficult hull style to setup. Tunnels are very sensitive to setup and more difficult to drive. Do you have the blue or gold sv motor? It's a nice size hull, with that motor look around a grim 42x55, 40x52/3, octura x440, x442, x640 & x642. Im not sure if the hull is going to like more pitch less diameter or more diameter less pitch. A prather 215 or 220 might not be bad either. It's gonna be trail and error bud, like I said tunnels are picky.