They can live in the same area because they're usually not in direct competition. They'll clean each other's clocks. Bottlenose dolphin will fight among themselves, and go after a common dolphin if it feels threatened. We don't see it happen a lot. But it occurs.
Dolphins are among the most lethal animals in the ocean.
They have excellent eyesight. Better than that of humans. They'll look for buoys, islands, things on the coastline. They send out clicks of sound that bounce off things, telling them things like how fast fish are moving, and in what direction. They see with sound the way we do with light.