I found a 30PIN POWER CABLE WITH WATERPROOF BACK DOOR HOUSING FOR GOPRO HERO3+&HERO4. The KAPAXEN product is no longer available with the waterproof back door. It is available without the backdoor and just the 30 pin Power Supply Connector. The cord is the same and will power it the same.
I have a KAPAXEN 30 Pin Power Supply Connector with waterproof backdoor for my GoPro 3+ and GoPro 4 that I use all the time. I have the one that is through the back and waterproof. The The USB connection is not completely waterproof, but can be protected to be outside. I have just not tried to put it under water. I have had it out in the rain with an extension cord cover and it worked great. I do time lapse photography with my GoPros and it will sometimes be plugged in for several days at a time and outside in the weather.
Same GoPro 30 Pin Connector without the back door. This will still come into play but may need to be modified to be waterproof. Click on either image to see it at Amazon.com
This link is to a list of the best selling power banks on Amazon.com. Make sure and look at the mAh value as the higher the mAh the longer it will last.
You can use the 30PIN POWER CABLE WITH WATERPROOF BACK DOOR HOUSING FOR GOPRO HERO3+&HERO4 in conjunction with a power bank. Using a power bank will drastically increase the situations that you can extend your battery time. Getting the GoPro 30 pin connector with waterproof back door will extend your ability to leave the gopro oustide or even submerge the gopro under water.