Here’s a little overview the set-up I use to charge my GoPro batteries while on the water. While using my GoPro, the dead batteries are charging. I can repeat this all day until the power bank goes dead.
Dual GoPro Charger: http://www.amazon.com/dp/B01LZHRYJS/?tag=l06aff-20
Extra GoPro Battery: http://www.amazon.com/dp/B01LWKCIQN/?tag=l06aff-20