I'm not at all certain A6500 is worth the price premium over A6300. Its main advantages are IBIS and touchscreen, neither of which are of any use underwater, so you end up paying $400+ more for the exact same sensor and AF tech. Even for land shooting, that's a tough bargain, unless you shoot a lot with non-stabilized lenses. Underwater, a strobe or a video light is a much better way to spend $400.
Agreed, with respect to underwater photography, if that's all the camera will be used for. Above water, I think it depends on what you shoot and what you shoot with.
Almost all (non-telephoto) full frame e-mount lenses, as well as all(?) non-Sony-brand APSC-C e-mount lenses don't have OSS built in (plus, there's unstabilized adapted lenses). So without the IBIS in the a6500, you've often got no source of stabilization unless you severely limit your lens selection. That may or may not matter to someone. If you don't care about shooting mostly static subjects, handheld, in poor lighting, without using excessive ISOs, then by all means save the money. Personally, I run into those situations a lot -- inside buildings (especially churches, temples, mosques, and historical buildings with poor lighting) or cityscapes and street photography at night. Also, a lot of people don't feel like IBIS+OSS is better than just either IBIS or OSS like Sony advertises, but I do -- at least situationally. Some people have surgeon-steady hands, but I clearly don't, and maybe that has something to do with it.
The touch screen makes moving the focus point easier, which is also situationally useful, above water. Sony desperately needs to completely overhaul it's entire user interface, but until they do the benefits of a touch screen are limited.
You make a really good point about lighting. Having better lighting -- when you can -- is a better solution to all of this than stabilization. Well, not the touch screen, but the rest of it.
