I have a RX100M4 and I debated the WWL lens. It is significantly more expensive, but also heavier, and you will need a buoyancy collar. IIRC, it is the only lens that offers full zoom through capability. Zooming in or cropping the image does not bother me considering the weight savings. Though if you have money to spend and don't care about the weight or bulk, go for the WWL1, it is supposed to be excellent.
Manual white balance is the best way but the Sony WB maxes out underwater compared to other cameras like Canon or Nikon, etc., so shooting in RAW and doing a little post processing may be required. Your best bet to counteract this or minimize it is to use strobes as
@aviator8 mentioned. I vote spending less money on the lens and forgoing the WWL1 and spend more on the strobes. Get as much light as you can afford. Having a wide angle wet lens will allows you to get closer to your subject and maximize more light as well, and reducing the distance, as mentioned before, will help with colors.
I recall there not being any filters that are compatible with the camera while also using a wide angle lens. Perhaps someone can correct me on this or provide other info.
Here are pictures from my most recent trip to Raja Ampat using the UWL-H100 with the dome and 2 YS-D2 strobes.
Amazon Photos