I think we (as cruisers) would probably be more concerned with Coastal water
depths, rather than with world oceanic depths (see below). Unfortunately, the scale of such a world coastline chart would be totally unworkable.
Practically speaking, your draft
of 6 ½ feet will be considered “deep” for many favourite coastal cruising areas. However, many shallower coastal waters can be safely (though inconveniently) piloted, by playing the tides.
Oceans cover about 140 million square miles (362 million sq km) ), nearly 71% of the Earth's surface, to an average depth
of 12,200 feet (3,720 m), with the deepest sounding being 36,198 feet (11,033 m) in the Mariana Trench (Western Pacific Ocean).
There are shallow seas around most continents, that cover gently sloping areas called continental shelves, which reach depths of about 650 feet (200 m). The World has over 882,000 nautical miles
(1,634,700 km) of Coastline
* Measuring coastline is a fractal problem.
ie: You could measure a coastline with a stick 1km long, get an answer, then try one 1m long and get another answer, then one 1cm long, 1mm long, and so on. Each time, the "answer" would be longer - notwithstanding the difficulty of deciding where the actual boundary between sea and land lies.
Canada's coastline is the world's longest at 131,553 nm (243,792 km or 151,485 mi.), including the coastline of the country's 52,455 islands.
The coastline of the U.S.A. is about 10,755 nm (19,924 km or 12,383 mi.).
There are over 175,000 square nautical miles (600,000 square kilometers) of coral reefs
in the world’s tropical and semi-tropical seas.
Tidal ranges are usually small in the middle of the ocean, but can be very large where tidal waters are funneled into a bay or river estuary. Hawaii
has hardly any tidal range at all, while the water
in the Bay of Fundy (Canada) has a range of about 40 feet.
Because the survey
vessels, that gather hydrographic depth
data, have great difficulty maneuvering near-shore, the “traditionally” charted depths are often less accurate for shallower waters. In recent years airborne bathymetric surveys have gained considerable commercial
success in the mapping of shallow waters. Satellite radar
data is also being successfully used in many parts
of the world, offering an alternative, faster and lower cost way to map large areas of shallow waters.