iPhone 13 Pro with Waterproof Housing, vs TG6?

I feel that all that discussion about sensors and lenses is missing the forest for the trees - the single most important differentiating factor is that TG-6 can sync with strobes, and an iPhone (or any other phone currently on the market) cannot. It really is that simple. Strobes are the single most important tool of underwater photography; everything else is secondary. Yes, a highly skilled user can sometimes, in very optimal conditions, take some natural photos that will approximate the quality of a strobe-lit shot - but those opportunities are few and far between. Some types of shots, such as sunballs, simply cannot be taken without strobes. Likewise for critters that are light-shy, will hide in dark crevices, and flee if you shine a constant light at them.

Therefore, I would put the choice like this: if you're content with low to mediocre quality shots taken with minimal effort and investment, either a phone housing or a TG-6 will serve you fairly well. If, however, you're seeking quality photographs, then start with a good pair of strobes, and then look for a camera. TG-6 is a very good choice for macro; it can be okay for wide-angle, given an appropriate wet lens, but it doesn't shine there. Its main limitation is not RAW (Craig must have it confused with Sealife DC2000 or some other camera), but lack of a fully manual mode - in general, people shoot it in aperture priority, and utilize various tricks to make the camera choose the appropriate shutter speed. If that is a deal-breaker, then you can look at Sony RX100 series, Canon G7X, Olympus E-PL10 (it is served by a relatively very affordable and well-featured AOI housing) or something higher-end.