Do I Need Special Growing Soil for Aquatic Pond Plants?

I like your comments David.  Let me tackle it from another direction.  Yes, you can use your garden soil in your planting containers and crates, but it is important to keep some simple factors in mind that will help set you on the path to success.
You want the soil to be porous yet firm with a good clay content.  Top soil that is rich in natural organic matter and well cultivated is ideal with a higher clay content.  If the soil is to sandy with a high silt content it will not hold in the water and will not provide a solid bed for the roots.  If the soil is too heavy and not balanced it will compact and restrict the development of the roots.

Here is a great little soil test you can easily do to test the balance of silt/sand, clay and organic material.  Dry out a soil sample until is crumbly.  Find a clear glass container and fill it half way with water.  Then add the remaining of the space of the dry soil.  Place a lid on the jar and give it a good shake.  After it is all stirred up let it stand until the soil separates in different levels.   The sand will settle on the bottom, then clay will settle on top of the sand and then there should be water with a layer of organic material on the water surface.  You have just simulated how the soil will react your pond. 

You do not need a high content of organic material as it will float up to the surface.  The goal is to have a layer of clay that is double the depth of the sand layer.  The clay will take the longest to settle down of the three layers.

Garth Epp Publisher, BuildingMyPond.com

You can but dont need to. Everyone seems to have a different take on this subject. This is what I do and works very well for me . In the pond I put untreated  kitty litter (clay) in the bottom of the mesh basket or pot then the plant which usually comes with soil for the retailer and top off with pea gravel. For taller plants I use a wider pot/basket and and line the bottom with 2inch stone as a conter ballance wieght then the same as above. In my bog garden /bio filter there is 12 inchs of 2in stone (to allow water flow and Bio bed) . I just make pockets and put plant and heavy clay soil into.  The biggest things to avoid are potting mix and anything that is light and airy as it will just float away and clog up your filters. Also always fully soak any soil before putting in to pond or even heavy clay will float and make a mess.