Setting the whole DSLR vs mirrorless argument aside for a minute, there’s one thing that’s remained constant amongst all of them. Don’t let it get in contact with saltwater. And if you are going to head out into the ocean with it, make sure to check that you’ve got a reliable dive housing with no leaks!
Lensrentals recently had a Fuji GFX 100 out, which they were told was used in a dive housing and “suddenly died for no reason”. It turns out that there was a reason. Not wanting to risk killing a $10K camera themselves, they spoke to Fuji, who gave them the “repair not possible” line. Well, if Fuji’s told you it can’t be repaired, and the insurance has already paid for a replacement, why not tear it apart and take a peek inside?