The numbers indicate the bottom depth below Low Water Datum (IGLD 1985), which is at 577.5.
In order to estimate actual depth, go to the Lake Michigan/Huron Monthly Water Levels page.
The left column shows feet above or below the Low Water Datum. Find the current month level and add or subtract it to the number on the soundings chart.
For example, if the soundings chart shows a water level two feet above Low Water Datum, add two feet to the numbers shown on the chart.
Here is a link to water levels of all Great Lakes: