Thanks to both of you!
For the calm water shots I filmed in 4k at 60fps, in the turbulent water shots I filmed in 1080p at 240fps - I didn't change any other settings from auto/default.
For editing I'm using Adobe Premiere - I create the video at 4k 30fps, so I slow the 4k 60fps footage down to 1/2 speed, the 1080p 240fps goes down to 1/8 speed (I also scale that up to 4K). I edit the contrast/vibrance/saturation on a per-clip basis, and I used the picker tool to select gray/nuetral objections in the clips for color correction
Yep, I'm using the selfie stick that I bought to use with my old DJI Osmo (original).
Yes, I'm just using the bare camera - I didn't realize there might be potential problems with sudden immersion/heavy splashing. So far using it like that in that hasn't caused me any issues - might be a good idea to get the housing though if I'm going to be using it like this going forward.
I'm shooting blind - just filming quick and dirty, though it might be fun to put the Osmo phone holder on the selfie stick so I could use the camera as a sort of periscope for underwater viewing.