I dive with a Sony RX100 II, housing, and strobe. It does indeed get a lot pricier when you start looking at interchangeable systems (I used to have a m4/3 setup). If you're just snorkeling, a Chinese housing by Seafrogs would be affordable for your A6600 and won't really test the limits of the depth rating. I've generally heard good things about these discount housings even for diving though. All underwater housings are a risk to some degree.
Some other things to be aware of with housings and underwater photography:
- If you're just snorkeling, anything below 15ft or so will have very little red light. Just to set expectations. Shooting RAW or adding a red/orange filter will give you a little latitude to WB but again expect blue images. The only way to get around it is to use strobes/video lights to add light at depth to expose true colors. But that's a challenge for diving, not snorkeling. I would definitely not recommend strobes for snorkeling. On camera flash is mostly useless because of backscatter.
- If you're buying a housing for your A6600, a flat port will magnify the field of view of your lens by about 1.25x. Ultrawide lenses inside a flat port will have corner smearing. A dome port will give you the native field of view of your lens in the housing, but you generally need to make sure you're matching the port to your lens.
- With a flat port, you can add a wet wide angle converter lens on the front to give you an ultrawide view. I use my ultrawide wet lens 95% of the time. Unless you're shooting macros, you don't need to zoom.
A GoPro is much better suited for snorkeling IMO. Underwater housings are large and more difficult to swim with. You need to be aware of the housing's buoyancy if you want to dive down below the surface. A small camera will be easier to manage getting in and out of the water. Less worry about banging front ports on coral or rocks, which will scratch immediately with poly. And no concerns about a housing flooding and ruining your main vacation camera.
GoPro's also have ultrawide lenses, which let you get closer to your subject to improve visibility and frame up more dynamic scenes. Much better for video. From what I've seen, image quality is on-par with today's smartphones which is pretty dang good for how small they are. The Hero10 does 5.3K 60p video, which you can grab a decent 20MP still from. I'd say get a GoPro and enjoy the scenery.