Entrapment can be avoided by staying away from strainers, sieves, undercuts, and proper swimming techniques if you come out of your boat. Don't try to stand in fast current up to your thigh area, chances are you will get swept downstream out of control. If you have an older boat, make certain the walls are very secure so it is unlikely to collapse. When purchasing a boat, make certain it has a full bulk head for your feet instead of foot pegs that were common on older boats. Make certain throw ropes are stored properly and carry a rescue knife in an easy to get at position on or in your PFD. Just like skiing, stay in control. If the run is too difficult, portage the tough rapids or walk out if necessary. Here is a good article describing foot entrapments: Foot Entrapments.
Extrication often requires hands on contact by a rescuer (e.g., two+ person wading techniques, live bait)
A good rule of thumb is when the current is strong, swim instead of wading when over knee depth. Entrapments in fast current are very dangerous indeed. Boaters can only hold themselves upright for a very limited period of time before their upper body strength gives way. For these reasons, rescues often involve getting a boat or person out to the victim ASAP which may place them in danger.