anything over 10-12 foot should be mode 2 after you update the firmware. if you don't update then ice mode should be used in over 10-12 foot of water.
ice mode and mode 2 is the normal return gain of the unit and mode 1 or open water is a lowered return gain. you'll see what it does when your using, just remember it's there.
in shallow water you will see reds change to yellows and much less red line(s) while in open water/mode 2 and when you switch back you'll see it act like the gain was cranked way up. this means in deeper water you want the return gain not lowered so the normal sensitivity is in play. if your in deep water and you forgot to switch is to ice mode or mode 1 then you'll have the gain cranked way up and have a harder time seeing your bait if it can see it at all.