I was in the business for about 10 years, U/W video that is, and there are some kinds of trial and error that may be more effective than others. I would suggest that you go to one of the local colleges that offers video production as a course and get a text book. It will point out what looks good and why and what the mortal sins of video / film are. This will help you compose the shots in the water, otherwise you may have lots of great footage that won't tell the story you are trying to tell.
The other thing that will be helpful is to learn how to completely disconnect the camera from your swimming motion. Part of this is the universal socket wrist that isolates the motion and vibration of your body from the camera. The other part of this is knowing what is framed even when you aren't looking through the view finder, that way you can look around, set up the next shot, not miss the action while you are making tape.
The college can teach you editing on various levels and once you have the basics down, learning a new computer based system is managable on your own.
Good luck,