In general, drag increases as the square of the velocity. To double your velocity, you need to quintuple the power. This is true for ships, however, small boats behave differently.
Drag on the hull can be segregated into three components: Skin Friction, Form and Wave-making.
At low speeds, skin friction and form drag dominate. At moderate speeds, depending on the fineness of the form, wave-making drag begins to dominate. As velocity approches "hull speed" (approximately 1.2* sqrt(waterline length) - varies depending on the fineness of the hull), wave-making drag is the dominant force. This is the point just prior to a hull planing and is analogous to the sound barrier in air.
Once a hull "breaks the sound barrier" and begins to plane, wave-making drag drops away and becomes almost negligible. Most motor boats have steps and strakes in the hull to facilitate separation of the water from the hull and achieve planing at a lower speed and reduce skin friction at higher speeds.
As speed increases, air resistance begins to exert more and more influence and becomes a major component of total drag (hydroplanes flipping over).
