For starters, the contact between the water and the boat's hull produces friction. One knot corresponds to roughly 1.15 miles per hour. Some racing sailboats can reach speeds of 30 to 40 knots. Calculate that range in mph. Motorized boats have raced at 200 mph.
Water currents can speed up or slow the boat, depending on their direction.
The faster a boat moves, the larger the waves it creates, and the more wave resistance it encounters. Then there's the matter of dealing with the wind, which frequently shifts. The technology driving racing boats is moving as fast as the boats themselves. Reichel should know, having designed boats for the last three America's Cups. He has seen a host of changes over the last decade, starting with the sails these boats use.