Ocean Planet

Ocean Planets are planets mostly or entirely composed of water. They could feature a rocky or icy core, with oceans hundreds of kilometres deep. They would occupy the the H2O band of the Mass-Radius graph, and would have densities of around 1 g/cm3.

These planets could potentially form from the bombardment of a rocky planet by comets, or the inward migration of an ice planet.

Ocean planets would have much thicker atmospheres (composed of water vapour) than silicate planets of equivalent masses, producing a much stronger greenhouse effect than on a silicate planet of equivalent mass. Low mass ocean planets would have relatively thin atmospheres in which water could evaporate more easily and, theoretically, could have much higher waves than their more massive counterparts.

Worldbuilding in Practice
"Dagny is a 'super-earth' ocean planet of mass 2.3 M⊕ and 1.9 R⊕, orbiting 0.51 AU from the K star Ascott. Comprised largely of water with a small silicate core, Dagny has a thick oxygen-water vapour atmosphere which maintains a pleasant 23° C on the planet's surface and deflects the worst of Ascott's radiation. While colonists are required to live entirely on boats and other floating structures, and also require the constant use of breathers outside contained environments, it is a small price to pay for a 'shirt-sleeves' habitable world with bearable temperature and atmospheric pressure, teeming with small, relatively non-aggressive life that forms the backbone of Dagny's aquaculture economy."