I guess that Jim is pretty proficient at color grading. Since you are not and it takes quite a bit of practice, get yourself a red filter. They are relatively inexpensive and provide excellent results. 90% of the good looking UW GoPro videos out there are done with a red filter. Trust me, it will make color correction way, way easier.
I use a better camera than the GoPro and still need a red filter myself.
Actually, I'm not that good... I used the tool to set the white balance based on some white sand in the picture (just like I do with my terrestrial photography). From there some minor tweaking (brightness and contrast) and it was done.
As Pajjpen said, I did my edits on a Mac and used iMovie, or Photoshop CS6 or GoPro Studio. I was able to get it adjusted (slightly different with each program but non-the-less pretty good) on my Mac. Then I tried it on a PC, and couldn't get the colors adjusted properly. The interface was slightly different as I recall (it's been a couple of weeks) but it's the same program. There's no reason I can think of that it wouldn't work on both O/S's the same.
I think that's the frustration Pajjpen is having.
Jim