1. A system for providing advance warning of underwater navigation hazards that threaten safe vessel passage, the system comprising: means for providing stored charted data generated slope profile information of the sea bottom; means for providing vessel generated slope profile information of the sea bottom; means for comparing the stored charted data generated slope profile information with the vessel generated slope profile information; and means, responsive to the comparing means, for determining whether the slope profile in front of the vessel is consistent with the stored charted data generated slope profile information.