Just to add to Rickies comment on debris in the water affecting the image.

A lot of people miss that you can quickly cycle thru your frequencies just by pressing the little check mark button rather than going through the menus, so you can see the differences.

I like to hunt on 455 hz at 75+ ft out and if I find something interesting will swing back around and press the check mark button to bump to the Mega frequency for a more detailed view.