Unfortunately there is no one size fits all solution, the tough cams other than the Olympus such as the Nikon and Panasonic are somewhat popular because they are rated to 30m without a housing while the Oly is limited to 15m, but is offered with a reasonably priced housing rated to 40m - however the cameras really are consumables without housings if you read the fine print in the Oly manual it tells you you need to have the seals replaced annually. I bought my daughter a TG-850 which had the seals fail once within warranty and a second time out of warranty, now a TG-4 and purchased the housing for it. The seals under the battery doors are accessible and you can inspect them but they seem to have a membrane seal under the control buttons which fails. I know a few others who have had the seals fail in a similar manner after a year or two of use. A housing has an o-ring you can service and all the controls are accessed through the sealed buttons/shafts, so it's a more durable proposition.
The TG-6 has its limitations but is a great macro camera without needing wet lenses, though once you get out of the shallows you really need a light source - either a strobe of a good video light as pushing the ISO will make the noise quite bad quite quickly and the shutter speed drops to unusable levels. It has two apertures f2/2.8 wide and f4.9/6.3 zoomed in, f8 wide and f18 zoomed are achieved with an ND filter It has no shutter priority or manual exposure. If shooting macro with lights that are bright enough this should not present a problem, there have been many discussions on dealing with this. Your mentioned humpback whale would ideally need a wet wide-angle lens as the field of view of the TG-6 (and all other compacts ) is quite narrow and require you to back off a long way to fit the subject in the field of view. Same for reef scenes, schools of fish etc.
I notice you also mention tech-dives - that is beyond the rating of any of the un-housed action cameras and also beyond the rated depth of some housings. So you would need to factor that in.
As far as competition goes there are very few compact cameras offered these days, lines like the popular G16 have been discontinued, the super zoom cameras are really not suitable for UW use as the lenses extend so far, mobile phone cameras have killed them off. Everything is moving to 1" compacts like the G7X, G9X RX100, LX-10 cameras and they do offer more flexibility than the TG-6 but will need wet lenses for real macro and wide angle. Snapshots are of course possible on medium sized subjects without wet lenses. And of course more $$. Something like a G9X in the fantasea housing might be the cheapest option there maybe $1200 in Australia +/-.
