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.
Another highly sought-after, and relatively exclusive, feature within the video camera realm is the ability to record in RAW or ProRes. These are uncompressed formats that maximize the information gathered by the sensor and provide you with extended flexibility when it comes to color grading and post-processing. As mentioned, this feature is quite an exclusive one and only available in a select few cameras on the market with good processors (again, excluding high-end cinema systems). Some select cameras can record in ProRes Raw with an Atomos Ninja V external recorder. Nauticam offers a housing for the Ninja V for use with your underwater system.