Modern smartphones are able to handle a bit of water. Most battery packs aren't. NiteCore's NPB4 is an exception. It's IP68 rated against water intrusion and drop-resistant, and although it weighs under a pound and is just four inches tall, it packs in 20,000 mAh of power with a maximum output of 18W. Outputs include two USB-A QC 3.0 ports and a USB-C port; the same USB-C port can be used as an input, as can the backup Micro USB port. It arrives with a USB-C charging cable as well as a nylon holster.
Length: 4.07" / Width: 2.02" / Height: 1.95" / Weight: 13.3 oz.
Capacity: 20,000mAh / Output (USB-A): 5V=3A, 9V=2A, 12V=1.5A / Output (USB-C): 5V=3A, 9V=2A, 12V=1.5A / Output (Multi Ports): 5V=3A / Input (Micro-USB): 5V=2.4A, 9V=2A / Input (USB-C): 5V=3A, 9V=2A, 12V=1.5A