My 2 cents is that the EMmkIII is a significantly better camera than the EM10mkIV, just because of the PDAF focus system and because the M10 series has really become watered down compared to the feature set in other OMD bodies. If you want to stretch a bit, the M1mkIII has dropped in price significantly on the used market since the OM-1 came out, and the AOI housing has a much smaller form factor than the one for the M5mkIII (price difference is just $100). Only downside to the M1mkIII and OM-1 AOI housings is that they take PEN ports instead of the OMD ports (without an adapter). This may not be an issue for you for macro and general stuff, but it gets kind of kludgy if you want to shoot Pro lenses like the 12-40 and 8-25. As far as the bodies, topside ergonomics and build quality on the M1mkIII are worth the difference IMO, plus you get some additional features that the M5 doesn't have.
Source- the last several months of me going back and forth between the M5mkIII and M1mkIII for underwater, ultimately landing on the M1mkIII. I came from the M1mkI in the Olympus housing, and I have to say the housing size difference is noticeable and appreciated. That said, any of these choices are much more bigger than the TG, and that's likely to have a significant impact on your diving. With a TG, you can be a diver who takes pictures, but with the interchangeable lens cameras and everything else that goes with them, you become a photographer who dives.