The RED cameras perform their best as a cut between regular cinema and documentary film tool. Configured either as a one-man-band unit or a complete studio package with all the bells and whistles, it handles most productions. Although shooting on RED may be the best option when it comes to acquiring maximum resolution in a small package, not everyone puts their faith in this camera.
Optical resolution is debatable. Many cinematographers (and audiences, subconsciously) prefer dynamic range and pleasing tonal qualities over miniscule details in on a pixel level. That is why cameras from Arri still rule cinema screens, almost 100 years after the start of Arnold&Richter Cine Technik. Arri Alexa is the dominating choice among discerning directors and DoP's, and many studio and controlled location productions prefer the Alexas. So when shooting these types of films, the Hydroflex housing is the choice for underwater filming.
However, there are a number of limitations with large format cameras. Namely bulk (weight and size), operational cost and sometimes low light performance. Macro shots can be difficult to obtain with large housings, and transport logistics and cost are sometimes a major consideration. There is also added time in post production where 6K may be overkill for regular HD mastering. So RED or Alexa is not suited for all types of productions.
For faster turnaround time it is possible to shoot straight to QuickTime Prores or via HD-SDI feed to the surface, especially useful for episodic television or live sporting events.
The smaller underwater housings and cameras can be transported as regular checked cargo and even as carry-on. This means fast and inexpensive shooting anywhere in the world with minimum delay. Carnets are in many cases not necessary. We can also bring two or three cameras for added backup in remote locations.
Interesting fact: Coupled with the Nikonos 15mm wide angle lens, the tiny Sony A7sII can deliver 4K images with about 15 - 20% greater optical resolving power than high end cine optics on RED Epic behind a coated glass dome (we have the tests to prove it).