The Newtonian gravitational constant G is a fundamental parameter of physics relating the gravitational force to the product of body masses by an inverse square of the separation. G has been measured with an accuracy of about 7 parts in 105 in individual laboratory experiments [e.g., Luther and Towler, 1982], but the consistency of all modern laboratory measurements is only about 7 parts in 104 (Figure 1), making it one of the most poorly determined physical constants of nature [cf. Cohen and Taylor, 1986; Gillies, 1987]. For the past century, virtually all experiments to measure G have been conducted on a scale (i.e., separation between test masses) of 50 cm or less, using a modification of the Cavendish balance. In recent years there has been increasing interest in determinations of G over larger scales than can be achieved in the laboratory. Some theoretical attempts to combine gravity with the other forces of nature predict the existence of a fifth force in addition to the classical gravitational, electromagnetic, weak, and strong forces. The fifth force would produce departures from Newtonian or inverse square law gravity at mass separations of tens of meters to tens of kilometers. Geophysical experiments are uniquely suited to measure the gravitational constant at these scales, and in this paper we outline the advantages of conducting such an experiment in the ocean.