Thanks for everyone's comments and general support which confirms my thinking is that too identical bodies/setups is hard to beat. As an aside ruggedness tests are not likely done by many intentionally but yesterday by accident my Z8 and 600mmPF took a hard drop from about 4 feet high and skidded along my drive way. After my heart started beating again I picked it up and the lens hood is scratch pretty bad, a small scratch on the hot shoe and after it was cleaned up I can find no other scratches dents or visible damage. I tested it throughly last night and I can find no functional issue at all. Please do not try this at home but hopefully it gives some comfort for others on the ruggedness.
No question the pressure is greater when diving and the dive cameras have to meet the minimum standard.I think you are right, the weather-sealing in a regular camera like the Z8 or Z9 is nowhere at the level of the Nikonos, but the latter are designed to be waterproof at what, 150 feet or more? Several atmospheres. Thus the sealing has to handle water under pressure, which is a much bigger challenge.
Cameras like the Nikon Z series are not rated to handle immersion.
In water the pressure doubles approximately every 33 ft. Unless you have special training and equipment a diver’s depth is typically limited to around 100 ft. You can go deeper but below 120 ft your safe dive time is very limited.
Open water depths vary considerably depending on the geology of the local area. It is not uncommon around me to find depths greater than 600 ft, in addition most channels for ship navigation are at least 50 ft deep.
If you drop your camera in open water it is going to sink. It will be soon at dive depths or worse. It is not likely to survive. You have to have it tethered and you need to quickly get it out of the water.