Why do speedboats go fast?

First, because they use high horsepower engines, and

second, because their hulls are designed for low water resistance.

There's several ways of doing that, but the most common is the "step hull" ... where when you get up to speed the hull rises onto a step (a mini-hull).

To see an example of this look up sea-planes; the pontoons (or hull shape of flying boats) are all designed this way.