Towing boats can be very effective but does introduce a higher level of risk.  I highly recommend the use of a proper rescue vest with a quick release tow strap along with the training to safely use a rescue vest.  These skills are taught in the Safety & Rescue Level 4 class, well worth taking.  An old school method that doesn't require a reswcue vest is draping aloop of climbing webbing with aq carabiner connection to the flipped boat's grap loop.  To quickly escape, simply dip your shoulder.  Towing is very fast and works best in mild current and pools. 

You can also tow swimmers while they hold onto their gear (or not).  When being rescued, it's real important to help out by kicking your legs for extra populsion.  Here is a nice video demonstrating this technique: Tow Rescue.