Considering the well regarded Sigma 18-50 vs the kit lens. Used market the Sigma is around twice the cost and has no OSS but is otherwise considered much better.
Sigma 18-50mm f/2.8, while an excellent topside lens, is not particularly useful underwater, as 18mm is too narrow to use on its own, but it can't be used with wet wide lenses. WWLs require the camera lens behind them to have its entrance pupil positioned well forward to avoid vignetting, and in fast zooms it is almost universally well inside the lens. A useful proxy for wet wide lens compatibility is the size of the front element - if it's small, like in 16-50mm kit lens, then a wet wide lens is almost guaranteed to work, whereas if it's large, like in the Sigma 18-50mm, or the Sony 16-55mm GM, then it's pretty much guaranteed that it won't.
Might also look for a very fast wide lens to do ambient kelp with the dome port.
A very fast wide lens is also not particularly useful as you need DoF to work inside a dome. I almost never shoot apertures below f/8. The only setup that I know of which keeps corner-to-corner sharpness at wide-open apertures is the adapted Nikonos RS 13mm fisheye, but it's extremely expensive (~$4500 for the lens, and another $1000 for conversion service) and requires a full-frame camera.
I would recommend either 16-50mm + a wet wide lens such as AOI UWL-09 or Kraken KRL-01, or a Tokina 10-17mm on Metabones/Sigma MC-11 adapter and a 6-inch or 8-inch dome port. Caveat about Tokina though, on an older body such as A6500 it can be prone to hunting - I use it on an A6700 and it's great, but I've read testimonies that on A6000 it was very unreliable, on A6300/6500 it's borderline decent, and on A6100/6400/6600 it's finally native-like (as long as you don't do video).
