Re: gilligans island
Reading a chart is much like reading a map, the lines of contour indicate a rise of the floor much like the contour levels of a topographic map
(elevations in feet mean sea level, north american datum 1927 transverse mercator projection)
but kinda of backwards.
(I should be looking at a chart map but only have a topo open)
While the height of water should remains constant, "BUT" due to tides and or spring run off if your in fresh water, it is up and down.
Pick up a book and do some studying is my suggestion.