I'm still learning the DI and SI game myself, but from what I've read and actually seen on my Lowrance 8 package, DO NOT be afraid to change the settings. Punch them buttons to change the picture in terms of color choices, contrast, range, etc. You can always change the settings back. Changing the palette color(s), contrast, etc., will either sharpen the picture you are seeing or make it worse, with no harm to anything other than another fingerprint on the HU. Changing the settings will/might make something appear that didn't when viewed or seen with the previous settings.
DI will not show "fish" like a 2d sonar will (either an arch or fish symbol), they show up as white dots and that's about it. DI will show the detail of the "structure" that looks like a blob (for lack of a better term) when in 2d mode. You'll see the individual branches/limbs/main trunk on a submerged tree/brush pile, or the individual stalks of weeds/grass, etc., depending on the location of the TD in relation to the object. Circling around for another look at something from another angle is always good, it may show more or less but you'll get a better picture of what the object is. The fish will then be a white dot in or around the object(s) being scanned. DI will also show contours in the bottom with much more detail than 2d will just because of the more narrow/stronger(?) beam it uses, where you should be able to see alot of the nooks and crannies that may attract fish.
DI will not replace doing some research on new water, look over the maps before you hit the water, so you'll have an idea of where the drop offs, creek channels, stump fields, etc., are and then you can focus more on finding the fish in those areas. Look over some of the pictures other people have posted, some post the 2d version and the DI version so you can see what the differences are between the two.
GO BIG ORANGE !
I meant to behave, but there were just way too many other options available at the time.