Water exerts a great deal of force on objects. The following chart is from River Rescue, 4th Edition - Les Bechdel & Slim Ray:

The force increases in proportion to the square of the water velocity. Water is heavy as well, it weighs 62.4 lbs per cubic foot (or 8.35 lbs/gal). Figure an average kayak holds 75 gallons without air bags, the total weight is: (75 * 8.35) + 40 (average kayak weight) or 666 Pounds. Now you can understand why we insist on paddlers installing air bags in their boats.

As you can see, these numbers add up quickly. If you are paddling in a small party like 3/4 boaters, you will may need some form of leverage to extract a pinned boat.