Instead of mph, boats use knots to measure speed. Because travel by boat is so fundamentally different from travel on land, knots are the superior unit when measuring the speed of travel by water. Travel by sea often entails much longer journeys than by land, where the curvature of the earth needs to be accounted for, so the current location and the distance you’ve traveled are calculated differently than they would be if you were on land. All of this must be accounted for when measuring the speed of a boat on the water.

Source: Lake Wizard