The Newtonian gravitational constant G may be determined from a comparison of gravity at the ocean surface and at depth, where the attracting mass is smaller by an amount that can be determined from the measured density of sea water. Uncertainties arising from instrumental limitations and localized gravity anomalies may be reducible to the point of obtaining an improved value for G, but it is any case of interest to verify that the result coincides with laboratory measurements on a scale 107 times smaller.