When it came time to purchase my own spear gun, I scoured the web trying to understand the differences between the various types of guns. I've found that there are three main types of spearguns:
The rubber sling spearguns are very popular throughout the world, and greatly vary in size, power and materials from which they're manufactured. Their main advantage over the pneumatic spearguns is the ease of maintenance -- sling-powered guns are simpler, and have less moving parts.
Here's a (very short) list of some of the rubber band gun vendors:
With pneumatic spear guns, the diver pressurizes the air chamber using a hand pump, which then uses the pressure to propel a front-loaded spear shaft. A properly functioning speargun can be pressurized once, at the start of the season. Since the compressed air serves only as a spring, and no air is lost during a shot, pneumatic spearguns do not have the bubble and noise problem of the CO2-powered guns. The advantage of pneumatic guns over the sling-powered guns is that they tend to be smaller, and more powerful. On the other hand, the air seals, hydraulics, and the generally more intricate mechanisms in the pneumatic guns mean that they require more service and maintenance than those that are rubber-powered.