It would be totally dependant on what you want to use the footage for.
If your using it for trip dvd's etc, then the HC9 is a great little camera. There are quite a few people on this forum that have used the HC9 with great results.
If you are planning on using any of your footage for broadcast, then you would need a different camera. The HC9 just can't keep up with the larger cameras.
As for learning, there are plenty of resources available on the web and this forum. If you have any specific questions, just post a topic and I'm sure you'll get plenty of help. In some cases, more than you can imagine. You will get conflicting advise from time to time. Don't let that deter you. You simply need to figure out which way best fits your style.
It's good to watch and learn from "pros", but you don't want to try to mimic any one persons shooting techniques or style. If you talk to ten pros, you'll get ten different ways of doing things. They normally all work, it's just how they learned or what they are used to.
The best thing to do is shoot as much as you possibly can, both topside and in the water. If you can't use the camera topside, then you're definitely going to have problems in the water. One of the main things to keep in mind is that you need to be a "disciplined shooter". Don't get in the water expecting to get 60 minutes of useable footage on a 60 minute dive, it just won't happen. Take your time and plan your shots and you will come up with much better footage.
Editing: Sony Vegas, Adobe Premiere, Final Cut...All good choices and you'll definitely need something for editing. I use Adobe Premiere Pro and love it. Others will swear by Final Cut, Vegas, etc. I would do a little research on each one and see which one will best meet your needs.