I love dolphins! There are 40 species of them. 
Me too! How do they communicate?
I am not too sure about that, but, they have well-developed hearing which is adapted for both air & water. 
How fast can they travel in the water?
Not 100% sure, but although they are not quite as flexible as seals, some can travel just as fast as seals. 
What do they eat?
Fish. They have conical shaped teeth that help them to capture fast moving prey.