The line is set on the governemnt charts (NOAA / COE) and again it is just a guide.
I never run on plane on the ICW unless I know I am in a deep section (10 to 12' min) free of trouble spots. The trouble spots are almost always when a creek or river creek crosses the ICW, especially near inlets. The stretch of water just north of charleston, behind isle of palm is a good example... You have a bunch of small creeks crossing the ICW and at almost every crossing, the bottom comes up. Usually not enough to be a concern but sometimes at low tide it can be
Whenever the sounder closes on 9' I back off from hull speed to idle. If it gets below 9', I go to slow idle and at 8' I go to neutral and start bumping in and out of gear. 7' and it s reverse time

. All that with a 6 1/4 draft...
Also, the bottom is rarely even so if it gets skinny, I usually change course a little to see what s on the right or on the left side of the channel. Usually you will see the depth change, slowly but surely... The key again is to go slow
But again, the number one thing is preparation. Every night, I review e next leg and check cruisers.net and active captain for the latest. The kettelwell chart book as an inch wide margin along each page on which I make notes for the next day
3 or 4 years ago, little river inlet crossing in nc was a real problem... On one trip I ran that stretch at night to make up some time and pass thru near high tide.... I had to poke three times on the north side to find enough water. Sure I was in the channel but on the first two attempts the bottom came up and I chickened at 7' third time was the charm with 8' thru. The plotter track told the story... Two aborted solid lines and finally one thru. Not a lot fun
