Everyone learns in primary school that the surface of the Earth is about 70% ocean and 30% dry land. But is there any reason to believe that this is the norm across the Galaxy? In this paper, Fergus Simpson argues that most habitable planets should actually be "water worlds," i.e., planets covered entirely with water.
Simpson's basic argument (and I'm oversimplifying here) is that there's no preferred ratio of water to dry land on a given planet -- it's just a random quantity for each planet. So planets with roughly equal amounts of dry land and water (like the Earth) are actually outliers -- they require a very lucky coincidence. (If you flip a coin 10,000 times, you are actually very unlikely to get exactly 5000 heads and 5000 tails -- it's more likely that one will exceed the other by a considerably amount).
So most planets out there should be either entirely covered by water, or completely dry. And since the latter are uninhabitable (except in SF novels like Dune), nearly all inhabitable planets should be totally covered by water. So the first aliens we encounter may turn out to be intelligent squids.