I just received my Sony a6500 and spent the weekend above water playing with it. I got the 16-50mm PZ "kit" lens and the 55-210mm Sony lens with it, with the 55-210mm lens solely for above water use (there's no port available for it as far as I can tell). So far the pictures look great and the camera is amazing (although there are so many settings to consider, at one point I went into information overload watching videos for all the settings "suggestions").
As for my underwater setup that I am building, I went with the Nauticam housing, 2 YS-D2 strobes, the port to fit the 16-50mm lens, and the Nauticam CMC-2 diopter and holder. The CMC-2 has less magnification and is way easier to handle than the CMC-1 if you aren't experienced in super macro shooting underwater. At some point I will get the Sony 90mm macro lens, but that's a bit pricey for starting out with this setup for now. Also, that requires a decent size dome, and I'd prefer to keep the setup somewhat smaller to start. I probably won't get the setup underwater until the summer, so for now, it's just going to look pretty.
Anyway, I know you said the Nauticam was out of your budget, but for now, I'm guessing the 16-50mm plus a diopter should give you some good flexibility to get going (I don't know your underwater photography experience).