The GH5s is popular for underwater video because the camera is at a great price point, the body is relatively small, there is a multitude of micro-four thirds lenses available for any lens requirements, the video bitrate is quite high, and camera supports many video features that other micro-four thirds cameras do not support. It crushes all other micro-four thirds cameras at high ISOs with respect to noise.
The Blackmagic Pocket Cinema Camera is one of the most exciting cameras on the market right now for video. It provides the average videographer with high-end recording options that were previously inaccessible with a small budget. For underwater video, the Blackmagic Pocket Cinema Camera with the Nauticam BMPCCII Underwater Housing is arguably the most enticing video system on the market right now.