"The in-game formula for this is 'Cavitation depth = 20 * SpeedInKnots - 100'. What this means is that for every 5 knots of extra speed, cavitation happens 100 feet earlier. "
http://gameplay.tips/guides/653-cold-waters.html
20 * 5 knots -100 = 0 feet
20 * 10 knots -100 = 100 feet
20 * 15 knots -100 = 200 feet
20 * 30 knots -100 = 500 feet