Good topic, thanks for sharing.
As for myself, I first started on a cheap compact (Lumix TZ10), but quicly switched to a GoPro4Silver (HD instead of 720p, really nice) in 2015, as I liked the image quality and had, after having done both on the compact, found out that I was not really interested in shooting stills.
I've been using GoPros ever since, starting with the GP4 silver, which was a really nice camera - if you do take the time to shoot in a flat profile and edit in post.
I first had a nice tray setup (homemade, then a SRP tray https://www.diveactiongear.com/product/187/the-tray-by-srp-for-gopro-hero4hero3hero3action-cam ), but when I actually started working in diving, I ditched the tray and simply had the GoPro on a coil retractor in a BCD pocket, and going for fully hand-held: Turns out you can get quite stable shots, even on the non-stabilized 4, just holding the camera with both hands.
I'd used a UR Pro filter on the Panasonic compact, and went for that on the GoPro, after watching "side-by-side"videos comparing different filter options, as I found it offered maybe less vivid, but also less "day-glo" like tones and more uniform results

Our faithful GP4S cams with the UR Pro CY filter.
While I wasn't teaching that much, I was still guiding most of the time so not that many opportunities to shoot - but with such a small camera always tucked in the BC when guiding, I sometimes had option of grabbing a few seconds here and there when it was possible do so so ( 10 to 20 seconds clips max, which works fine for editing anyway, when an experienced guest was taking time to shoot a picture, for instance) or documenting something unusual.
And I also still could do a few personal fun-dives every once in a while where I could shoot whenever I wanted, so all in all this kind of action cam rig worked quite well for the short, storyless, video logbook type clips we're interested in.
My wife was doing the same, and we collected some nice footage, advantages of being in the water everyday.
Switching from the GP4S first to the GP6Black, then to the GP7Black in 2019 was a massive relief because the cam was now waterproof, which made it way more compatible with the type of heavy-duty usage it was getting, ie carried around in a BC pocket on-up to 4 dives per day liveaboard...
Quality wise, I much prefered the old "medium" FOV that the GP4S had over the 6 and 7's "linear" FOV which had higher side distortion, and find the image quality / dynamic range had also dropped a little on the 6/7 (probably due to stabilisation?) but the 7 is still a very capable camera, offering an notable improvement in colour acquisition.
I don't use lights, for practical reasons (bulk), and also out of personnal aesthetic preference.
I'm well aware that this is far from a common opinion, video/photo being all about capturing light and all that, but outside of macro video, I actually prefer less vivid colours and/or a colour cast, but a more even spectrum, over the usual results given by a set of lights blasting away in the foreground - and ambient light is also more discreet...
This is of course linked to the fact that I was diving in the tropics, where it is possible to get workeable results down to 20-25 meters on a good day, and where most of the "good stuff" is in the shallows anyway - unlike say darker waters or freshwater lakes...
From the GP4S to the GP7S, I kept shooting in ambient light down to 20m, in a flat profile, with a UR-Pro filter (SRP Blurfix adapter), and working on the colours in post (FCPX, DaVinci).
Some clips here, all shot on handheld GoPros 4 to 7, in ambient tropical light in various locations:
For macro, I did buy a MacroMate-mini +15 diopter, but never really had a chance to use it. Nailing the focus distance is definitely tricky, and you get blurred edges on the 7's FOV, so I moved away from that, especially when my wife bought an Olympus TG5, which gives interesting results for macro video, despite the terrible battery life when filming. I do have a pair of cheap Archon DV11 supposedly 800 lumen lights for macro/night dive fundiives.
I'd read about the lens hack you mentioned, but didn't try modifying the lens position as I only one one workable cam - but did recently change the lens on the GoPro6, switching to a MAPIR PeauProduction 3.37mm lens lens https://www.peauproductions.com/ which allows you to use the "wide" fov without fisheye distortion, I've played around with it on land in lockdown, good times.
Interesting ideas for the tripod, I was actually planning on experimenting with the 3K Gorilla Pod that I have in the near future, on a tray. I played around in the pool with the lights + TG5, seemed to work ok.
Maybe it needs to be weighted down for stability? I really like keep the tent pole idea, easy to carry around.
I agree it's definitely possible to do nice things with GoPro-like cameras, despite obvious limitations.
It's a shame theses cameras do remain action-cams to this day, with most recent functionnalities cleary aimed at the action-cam market rather than more "traditionalist" uses such as underwater video, but hey, if what sells is clearly people wanting to "be heroes", I get it ( which is where the Paralenz market positioning is quite clever in my opinion, advantages of the action cam setup without the focus on action and POV shooting...).
As an example of great use of the GoPro, I'd like to add this promo video was shot by Alex Lindbloom exclusively on GoPro 5s and 6s (with the exception of the aerial drone shots), rather than his more common GH5.
Lovely angles and creative use of tripod shots, lovely stuff.
Otherwise there are a few really nice clips posted on these GoPro UW video FB group that I'm sure you're familiar with:
https://www.facebook.com/groups/2347204708888086
https://web.facebook.com/groups/849303718445927
I reluctantly had to archive the GoPro4S rigs for luggage/mobility reasons last year, and will now just keep the 2 GP7Bs I have and the lens modified GP6B.
I currently have no intention of upgrading these cameras to 8/9/10 as I'm satisfied with what I getting, and also recently bought a compact for video, for more flexibility (Lumix LX10 in a Nauticam housing see https://wetpixel.com/forums/index.php?/topic/65996-choosing-a-compact-for-underwater-video-only-lx100ii-rx100-vvi-lx10/).
Given the current worldwide situation and the impact on travel and tourism, after some years of moving seasonally around mostly between Indonesia, Mexico and Thailand, we'll be settling down on a remote Japanese island in the near future, which might mean the possibility to dive more for myself ( lots of shore-diving options, and won't be working full-time as an instructor-guide), albeit in slightly colder and darker waters...
If we like it and end up setting up a longer-term base there, this might lead me to invest in something bigger like a second-hand GH5 rig (once the GH6 is out for instance), and might be the end of the action cam adventure for me, but I'll definitely be keeping the "GoPros in the pocket" option as handy backups.
cheers
b