This will probably sound stupid but because our lakes have experienced so much run off and dirt sediment... Those dotted lines are like thermocline but it's really dirt nasty that's going to settle at the bottom. Notice in your picture how it clears up from 0-5 ft...
Set it to pallet 13. And i suggest leaving it on auto. Make sure surface clarity is set to Shallow water ( not fresh water ) and turn noise rejection to low or even off.
What lake were you on?