The white boxes are just "empty" areas with no sonar information. This happens because the recorded range of the sonar can change during the recording if auto range is selected on the sonar unit, so if you have areas of deeper range showing at the same time as shallower areas, then you will see some of these empty areas - they are not covering any actual sonar information. If you zoom in horizontally so that you are viewing a smaller time period then they will be less of an issue, but you will still see them as you transition between different recorded depth ranges.