What would happen if asteroid 99942 Apophis collides with the earth?
If it hits the deep ocean, not much relatively speaking. In the recent past, people wildly exaggerated the capability of an asteroid (at least ones in the size range of Apophis) to cause massive tsunamis; in reality, the ocean efficiently and quickly absorbs the impact. That said, it'd still be terrifying to see such an impact in deep water.

If it hits a city, it would cause violent devastation, immediately killing 10's or 100's of thousands with its initial impact, and millions with the resulting shock wave and debris.