Which speed and water depth does the manual specifically identify as enough for hydroplaning to start?

The manual says hydroplaning can start at speeds as low as 30 mph, well below the highway speeds this choice names. Treats hydroplaning as a highway-only problem, carrying over the mental image of standing water on an interstate instead of the shallow film the handbook describes. This answer treats hydroplaning as a highway-speed, flooded-road problem rather than reading the handbook's low-speed, shallow-water figures.

The manual names 30 mph as the speed at which hydroplaning can start, not a figure between it and highway speeds. Splits the difference between the handbook's number and typical highway speeds, trusting a rough driver's intuition instead of the figure the manual prints. Carries over the defensive-driving tip that pairs hydroplaning with a downpour at cruising speed, and picks a number that matches that mental picture rather than the one the manual prints.

The manual names water little more than 1/8 inch deep, far less than two inches. Assumes serious hydroplaning requires serious water, borrowing the rule of thumb from flood-driving warnings about six-inch and twelve-inch depths. Reaches for the depth that warns drivers against entering moving floodwater on a road, when the handbook's hydroplaning figure is about a film thin enough to miss on a wet morning commute.