Exoplanet is the current frontier of astronomy and planetary sciences. In this talk, I will present the evidence of abundant ocean planets and water worlds among exoplanets in our galaxy. Ocean planets are planets similar to our own Earth, on which water covers a significant surface area. Water worlds are planets with even more water, which may have ocean as deep as 1000 kilometers and possess bizarre phases of hot ices in their deep interior. Studying them brings insights on their formation, composition, and habitability.