Your latest image is a lot better. Are the white dots you are talking about the lighter areas all along the bottom transition from the water column? If so, you will tend to get more of that type of return on harder bottoms. Sometimes the contrast setting will tune those out or reduce them.

Also, make sure your transducer is parallel with the water surface. That will have a big impact on the quality of the returns. There are some good videos on YouTube for this topic. Search for "transducer level coin".