I've been asked how to do it and here it is. This doesn't mean you have to give away all of your best spots but I have found submerged islands and rocks and things that could damage a boat and always add those. This is what they look like on the app and graph.
First scroll to the area you want to add it, you will see it selected when you touch the area and then click the "?" and it will bring you to another screen. Then click "edit map"
Select what type you want to add.
Enter whatever you want to add.
This is the end result.
