Onboard a marine vessel in transit a speedometer, a fathometer and a computer run in conjunction to track the Froude depth number (which is a function of vessel speed and water depth). A warning (e.g., display, signal and/or alarm) appears and/or sounds when the vessel nears or approaches the critical Froude depth number value of one (which is associated with wake wash at its most deleterious level), thus alerting the crew that the vessel's speed must be adjusted (usually, reduced) in order to avoid Froude depth number criticality. Typical embodiments establish a Froude depth number threshold value below 1.0 (e.g., 0.8), above which a red (prohibitive) zone warning is given. Some embodiments additionally establish a Froude depth number value above 1.0 (e.g., 1.2), below which a red (prohibitive) zone warning is given, above which an yellow (cautionary) zone warning (appreciably different from the red zone warning) is given.