A marker float setup is used to find the dept of the water by casting out a float fastened to the end of your line with a weight sliding on the line. After you cast out you pull the float under water until it sinks to the weight on the bottom then you keep track of how much line it takes to let the float to come to the surface of the water. That is how deep the water is in that spot, some marker rods have a scale on them to help with the measuring. You continue to cast until you have marked out all the spots in your swim and marked them on your map of your swim.

Some times a marker float is also used to mark where you have put in a chum pile so you can cast to it every time.

Marking your area can be very helpful. carp patrol at certain depths. Knowing the contour can up your catch rate a lot if you know how to do it. but it does take a little while. However.... it takes a lot less time than fishing and blanking several times.

Once you know how the bottom is shaped, try making a thin line of bait that runs from the deeper water where you are fishing, to the shallow water right in front of you. Fish at two different depths on that line. Most artificial lakes has a channel in them, where the creek was before they dammed it. Along the edge of that channel is a good place to put one of your baits, and then one somewhere in the middle between there and shore. When fish start feeding, you will probably see them start hitting the surface. That will be an indicator of what depth they are feeding. If you see one crash, put bait right where you saw it.