Boy, I'm glad someone finally explained this. I never could figure out the formula between Lake Okeechboee depths, according ot the Army Corps of Engineers, and minimum depth on the Okeechobee Wateray, until now that is!

As regards the Lake Okeechobee depths, when you see a Corps of Engineer posting like this:

U. S. Army Corps of Engineers, Jacksonville District
Lake Okeechobee and Navigation Depth Report
Data Ending 2400 hours 08 MAR 2009
*Today's Lake Okeechobee Stage = 12.56
++Today's Route 1 Navigational Depth ÷ 6.50'
Today's Route 2 Navigational Depth ÷ 3.90'

The way you should interpret this is that the value given for stage (in this case 12.56 feet) is the height of the surface of the Lake against a datum, which is approximately sea level. This is an "NGVD" which is a  "National Geodetic Vertical Datum". (For more info just Google NGVD if  you wish).
So, this means that the lowest grounding point in the Route 1 is at 6.06 feet above sea level (or to be more exact the NGVD), and the lowest grounding point in Route 2 is at 8.66 feet above sea level. Thus, the depth of Route 1 and Route 2 is simply stated as the height of the lake level minus one or the other of these two grounding levels.
Ken Bloomfield