The monitor is a very important accessory for underwater video.
Of course, one can do without it. I myself did not have it for years and still brought it home. But if you want to improve, the monitor becomes indispensable. It is a matter of efficiency, maximum result with minimum effort. As I wrote some time ago: Shooting video underwater without an external monitor severely limits the ability to edit the image composition and in some cases it is simply impossible to shoot certain footage without damaging the underwater life around. This is the pure and simple reality.
All underwater monitors on the market are field monitors housed in a case. Most are sold as one piece except Nauticam and Marelux who sell very expensive housings for very expensive Atomose SmallHD monitors and recorders.
Staying with monitors, I believe you can save money and get excellent results with a one piece monitor and housing without spending a fortune on an Atomos Shinobi.
I have for years had a 5" Hugyfot monitor that is nothing more than a boxed Feelworld monitor. It has only 400 nits of brightness. Very little for sunny tropical surface waters. Yet until yesterday they were all like that and we survived.
BTW the new generation of field monitors are all at least 1000 nits. for land shooting I have a Chinese Osee T5+ monitor and it's really fantastic. Beautiful colors, bright, perfectly visible under the sun and lots of features. wish I could bring with me underwater. Be careful because over 1000 nits they have a fan and get hot and noisy.
Factors to consider:
Weight in the water. A seemingly bulky housing has the advantage of weighing less in the water and creating fewer problems for kit trim.
The routing of cables relative to one's housing. Actually, no one swim around with their monitor mounted like the Eiffel Tower on their housing, but sometimes the position of the bulkheads of the housing and monitor combined with the length of the cable make it difficult if not impossible to mount the monitor in the desired position.
Maybe with the help of other members we can come up with a rough list of models currently available on the market!
I'll start with an odd thing I noticed days ago.Some friends of mine use SUPE (Scubalamp) strobes with satisfaction, and I had seen that their new monitor was just introduced:
https://scubalamp.com/products/um55-monitor
Then yesterday by chance I came across a clone. Oh God I don't know who is clone of whom.
https://fotocore.com/products/monitor-mr6
Both sites are missing very important information such as weight in water and there is not a shred of user manual.