Olympus TG-5 vs Sony RX100 iii with underwater housing Sony MPK-URX100A

While I don't own a Mark III, I do have a Sealife DC2000, which supposedly has the same sensor and also has an aperture of f1.8 at 31mm equivalent, but fixed focal length. Compared to images I've seen from buddies who use TG-5s, I would definitely say that images are a lot more better given the bigger sensor size. Having however just done a couple first dives shooting with my full frame camera, I should point out that bigger sensor size also poses more challenges; in particular, I have to get used to the narrower depth of field and change settings and control bouyancy much more. Still, I think that going from 1/2.3" to 1" sensors was much less of a challenge than going from 1" to FF.

With regards to the TG-5, I found two issues that would put me off: No shutter priority (which Iˋm on most of the time because underwater sealife can be surprisingly quick) and the JPEGs really have overdone white balance (ocean water often looks purple from the harsh red filtering; but RAWs can fix this).

That being said, the TG-5 also has advantages: You do have a zoom lens, I love the form factor, and the camera itself is waterproof as well (on my first dive safari, I did accidentally catch something in an O-Ring in my Sealife housing; thank god that the DC2000 is waterproof as well and gave me another level of safety). Also, the macro feature looks amazing; with bigger sensors, you'll definitely will have to buy additional expensive (wet) lenses to have the same capability.

While I donˋt know much about the video quality about the TG-5, the contrast-detect autofocus on the DC2000 is atrocious: Diving with particulate matter or inside emecaverns will results in extreme and continous hunting (and noise). My guess is that the Mark III wonˋt fare much better.

Given the dive profile you mention, I would definitely suggest to first get decent underwater strobes (yes, more than one) and rail system to get much needed light which you definitely will need below snorkeling depths. With more light, you can add so much more features to your UW photography/videography; even editing gives me so much joy because revealing color features in RAWs is like rediscoving your dive once more!

Once you believe you have grown past the very able TG-5, you can for sure resell it or keep it as a backup or travel rig.