Ultimately, your choice in action camera should come down to performance and ease of use. We've filmed hours of footage with many of the major contenders to determine where each device stands in the field. Some excel in all manner of extreme situations, while others can fall apart underwater or once the sun goes down. And what good is an action cam if it's not built for action?
While the original Sony RX0 drew plenty of attention for its 1-inch sensor in a tiny body, it was somewhat hamstrung by the fact that to record 4K video it needed to be physically tethered to an external recorder. So while it was waterproof, shockproof and all that jazz, this limitation meant you couldn't record 4K video in these extreme situations. The RX0 II does away with this restriction, recording pristine 4K video internally, and also adds welcome extra features like a flip-out screen. It's the most expensive on this list, but if you need the low-light latitude a 1-inch sensor gives you, it's really an unrivalled prospect.
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.