The image quality difference is very minimal. Usually you can only see the differences if you zoomed in at least 100%. I would advise you to get into something like Sony a6400 and save your money for good lenses. The reason to go for sony is that you have the upgrade path from APSC to Full frame in the future. If you go canon eos M6II the mount is totally different from the EOS R and RP mount. OTOH, if you have the money to go FF body + lens, then go straight into FF. Any FF of those you've listed above is more than capable for underwater photography provided u can get the housing.
  If you want to get Sony, at least the a6400, the a6000 is really dated at this point. If you want to go for fujifilm, at least get a XT20/30 body. IF you really wanted the flip out screen, then the XT200 is fine. Just don't go for the XT100 no matter how cheap it is, unless you going to get it for free. The AF is really slow on the XT100.