" You are also looking for hard bottom, that typically shows in yellow color, soft bottom typically shows red."
Uncle Thomas is right...you are looking for hard bottom...
But I must differ on his info on the hard bottom color...most of the newer finders will let you set your color pallet for your display and with that said it could be any color...But...
The older original traditional and the modern default colors are usually Red for hard bottom and then the color travels up the color spectrum to Blue and Green for the softer returns on your sonar like small bait fish or squid.
I've been using Lowrance since I joined the Hobie team so most of my experience is with those units...
Besides the hard bottom or structure you should also be looking for bait or fish marks on the bottom or just off the bottom...they don't have to be big arches but sometimes it's just blue stuff as opposed to a plain flat bottom...do a few drops and see if it's just whitefish or actually some nice eating Vermilions...
Once you've found some biting fish don't forget to use your GPS bread crumb trail to relocate on them and if it's a really good spot...hit your MOB button to set a waypoint that you can return to on your following trips.
Good Luck...