In addition to Gamespot's "Lowdown" article, IGN have published a 2-page 'Swimming' feature.

• Moving the left analog stick forward triggers the classic 'Breaststroke' movement. If you hold down the 'X' button whilst doing so, CJ will switch to freestyle mode, and swim a whole lot faster. However, this can only be done for a limited time, before his stamina runs out.
• You can dive down under water by pressing the 'Circle' button. Again, you can hold 'X' to swim faster underwater, and witness the local marine life, before swimming back to the surface. If somebody is chasing you, swimming underwater will cause the pursuer to lose your trail.
• When swimming underwater, a Breath-o-meter appears. Should the meter fall below '0', CJ drowns.
• Lighting effects change in rivers lakes and other such areas.
• If CJ drives a vehicle into the water, he can walk back to land from any beach or grab onto a hedge, aslong as it's near enough to reach. (Providing that he escapes the vehicle quickly enough)
• Whilst in water, you can jack boats. However, timing must be perfect, and if you do it well enough, the pilot will walk off the edge of the boat in fear.
• Non-playable characters can also swim.
• The wave physics have been completely re-worked. If a storm is to hit San Fierro, the water becomes rough and choppy. The waves also affect boats, and may push them around a little.

IGN also released a few screenshots, check them out here.

Related Links: IGN, Screenshots, GTAForums Topic