We've considered the possibility that aliens could invade. Sometimes they fly, sometimes they walk, sometimes they sprout and take over our bodies when we're sleeping, but only rarely do they swim. There's a dearth of water-based aliens in media, despite the fact that statistics prove that in 60% of regular comic book continuity Aquaman's wife is an alien. These alarming numbers show that we need to pay more attention to how many exoplanets could have large oceans. To do that, we have to take a look at how oceans form.
Most scientists nowadays believe that earth's water was here from the beginning. The dust cloud that formed the solar system had water in it, but much of it would have been blown away when the sun ignited. Rocky substances can hold on to water well, and so when the earth started pulling itself together, a great deal of water came along for the ride. As the earth came together, it put more and more pressure on the stuff in the middle. Some water made it to the surface as more dense material sank down. Eventually, the central material heated up. This heat pushed water up and out through the earth to the surface. If the earth were an exposed rock, the water may have been lost, but the atmosphere kept it close and allowed it to rain back down.
So, for an earth-like planet to have an ocean of water, it has to have a certain mass - in order to build up an atmosphere - and be made of material containing a certain percentage of water. Not too much is necessary. Two thirds of earth is covered with water, but it only makes up 0.2% of earth's mass. Even half of that percentage can still bubble or steam up through the rock and make it to the surface. As for having an atmosphere that keeps the water around, an exo-planet would have to have between .5 and 5 earth masses to 'make it rain'. The overall process takes time, but an ocean may be formed as little as 100 million years into an earth-like planet's life cycle.
Sound pretty innocuous so far, but another few billion years after that and it's Invasion of the Oyster People. They've read The Walrus and the Carpenter, and they are not happy about it.
Image Credit: NASA-JSC-ES&IA