Please, please don't consider using CO2 in your sub unless you know exactly what you are doing with it, and the system is designed for it. CO2 is stored at very high pressures (800+ psi) and can be deadly. For comparison, Propel stores liquid at about 75 psi. If you were to try and "refill" your onboard Propel system with CO2, you'd blow it to bits, destroying it, probably destroying your boat, and almost certainly injuring yourself and maybe others nearby.
Having said that, I have been running CO2 ballast systems for almost 10 years, and they can be very reliable. They are most useful in larger boats, where you have the room for the extra componants required and safety devices, but also due to it's high expansion rate, you get more "blows" of your ballast tank per fill.
Several years ago, one issue of The SubCommittee Report (#35, I think) had numerous articles concerning the types of ballast systems, their implementation, and the inherent risks and safety requirements. I would urge you (and anyone getting into the hobby) to get a copy of this issue and read it before making any decision on a type of ballast system. Please use caution, and make informed decisions. We want you to be around for a long time!
Membership Chairman, The SubCommittee
Edited By subdude on 1058357793
Underwater, the ONLY way to fly!
SubCommittee member #0069 (since the dawn of time)