What is the practical minimum range for the Omniscan 450 SS

There is no actual minimum depth. The minimum range is limited by the near-field issue. The effective beam width gets wider in the near field, and side lobs get bigger. A practical OS450 minimum range is 2-3 meters of water depth.

As the effective beam width gets wider at close range, you may not notice much, but the closer you get, the more the image will blur.