Upgrading from really old DSLR Camera to a Mirrorless, but need advice?

So I guess I'm what you would call an amateur photographer. I've had a Canon Rebel XTI DLSR for years now w/ the kit lense it came with. But I'm thinking its about time for an upgrade to a newer camera.
I want to be able to be more advanced level than a regular point and shoot, but also can take video which could be used to create vlogs, and be small enough for travel. I want something to use for underwater photos so was considering looking into waterproof housing. Hoping to keep the budget under/around $800 lens included.

I came across the Sony A6000 and it looks promising. The negative reviews on the kit lenses are disheartening but it could be wonderful for me. The camera is decent priced that I feel like I could buy a Neewer/Meikon underwater housing (because I don't have the money for a Nauticam).it scares me about the possibility of it leaking and ruining my camera, though I have seen good reviews.

also interested in Nikon 1 AW1 due to the fact that the camera is already suitable for underwater, But I've read a number of reviews that claimed leakage and it ruining the camera, which makes me nervous.

I wonder if I'd be better off getting the A6000 or a cheaper mirrorless and buying a cheaper actioncam with underwater housing for any water photos on the side, like the Xiaomi Yi, though I feel like I'd be sacrificing quality of underwater photos. Even though I know the Gopro is good, it would probably put me over my desired budget if I got it alongside a Mirrorless.

Gjghh

Ironically, the manufacturer's warranty doesn't cover water damage when the camera itself fails to keep the water out.

The A6000 is good camera. However, you need to know what you current DSLR isn't allowing you to do. Then with that info, get a camera that can do what you need it to do. Getting a camera because it's more "professional" by itself will often leave you disappointed.
Better glass will always produce better image quality than upgrading the body.
You should look at the body as part of the system that provides the control that you need to capture the image. Going with a "better" body won't necessarily improve image quality.

For example, if your current kit can focus fast enough, then you need a better body.

If you don't like Sony's kit lens, then just buy the A6000 body only, and get the lens you want. Photozone.de has really good reviews on lenses that allows you to see just how sharp one lens is compared to another.

Leaking waterproof gear is often due to user error, otherwise you'd see a lot of complaints about the product. Nikon had an underwater film camera at one time. It was discontinued due to a massive amount of complaints of water leakage. Keep the gaskets clean, properly lubricated, don't exceed the max depth and you should be fine.

Since you don't list any problems with your XTi, maybe you should keep it and get the underwater camera you want, which is the GoPro. That will keep you in your budget.