Because boats take less energy to move through water than a car takes to move over land. So even a small amount is enough. But really, that's a lot of solar panels in the picture. You're looking at a kilowatt, at least. That's about 1.5 horsepower. I wouldn't be surprised if it's more than that.

The other half of the solution is the careful engineering of low-friction hulls. They cause the least about of drag as they move smoothly through the water. This is how a relatively low amount of thrust is enough for them to hydroplane.