Although I dive with a Sony A6000, my "main" camera is a Sony A7iii. Short version, I love it.
I switched from shooting Canon DSLRs for years to Sony last year, and I do not regret it in any way. The A7iii will serve you well above the surface. I have shot everything from Fighters flying through the "Jedi Transition" in excess of 400kts, to Milky Way shots in Canyonlands NP, and it has never let me down. At one point, Sony had a reputation for not having a good selection of lenses. That is no longer the situation. I have a Tokina 20mm f2 for my Milky Way images and the Sony 200-600mm f5.6-6.3G for photographing Birds of Prey and aviation. For day to day use, I have a 24-105 f4 G.
Keep in mind though that whatever camera you ultimately choose, if you do plan to take it underwater, you will need a housing and lens ports for it and these will often cost significantly more than the camera and lens did. Check out places like the
Backscatter or
Mozaik websites to ensure that your camera and lens are supported with housings
and ports.
Let me get up on a small soapbox for a minute. Keep in mind that accidents do happen. No matter how meticulous you are in your prep, your maintenance and the care you take of your o-rings, something you
never saw coming can cause a leak. With that in mind, always know that any dive with your camera could potentially be the one that destroys it. For that reason, I will never (again) dive with a camera that I can not afford to put at risk. That is why I dive with an A6000 but use an A7iii as my primary camera. Things like a vacuum valve go a
very long way to confirming that you have a good seal, but nothing is perfect. OK, I am done on my soapbox.
Depending on your budget and on your focus, you might want to consider a smaller camera such as a Canon G7Xiii or a Sony RX100 that have a (roughly) 20MP sensor and can shoot 4K video. The advantage of these are that you will get good (maybe not great, but good) image quality in a small package and will not require additional lens ports (although as you get better, you may want to add a close-up diopter and/or a Wet Wide Angle lens). With this type of camera, not only does your packing become easier since you will likely be able to pack the camera and housing in your carry-on, but you will be pushing a smaller housing through the water column. (The size and weight of a housing for a Full Frame DSLR or Mirrorless camera such as the EOS-R or the A7iii is quite significant.) If you get a high quality "compact camera & housing for underwater, then you may still be able to afford something like an A7iii or an EOS-R for your above the surface work.
FWIW, I won't recommend one brand over the other. Both Sony and Canon make excellent cameras at just about any given price point.
If I was in your shoes, I think that what I would do (and keep in mind that this is just
my opinion and not hard fact) is to get the
Canon G7Xiii + FantaSea housing (with vacuum valve) package ($1249.95 - which is less than most housings for the A7iii would be) to dive with and then an A7iii (or EOS-R) as my primary camera.
