Sony Alpha a6300

Pros

• Congrats, I did the same just recently but with a used Sony a6300, also with the Intention of talking it scubadiving.
• With video requirements rather look at 2nd hand mirorless like Sony a6300
• I started with the a6000 a lovely device, currently on the a6300 also great.
• I bought it a few days ago, since I have changed two cameras I had, a Canon 4000D and a Sony Hx80 for this Sony A6300
• The A6300 is your best bet here, especially as a kit. It's a great value for money camera, and you'll get really great results, even whilst learning.

Cons

• Sony A6300 with kit zoom.
• A6300 is A6400's predecessor, three years older, with a weaker AF than the other two.
• I’m getting tired of working within the constraints of the a6300 and I am also (admittedly) falling for influencer marketing on social media. 4k 120, 10 bit, dual slot sd etc are nice and integral upgrades that the a6300 doesn’t have.
• Had owned it before and loved its size, but changed it for a 6300 because of lack of evf.
• It seems to do quite a bit of focus hunting with continuous Autofocus in video mode only (also on my A6300)