Gazing beyond our atmosphere, we see water in space, too. Comets are largely balls of ice. The moons of many planets are globes of solid ice, nearly to their cores. Most of the night's stars stand enveloped in gaseous water vapor: steam! Water in all its forms recurs like advertising circulars sent to every isolated galactic hamlet. Water is the most common compound in the Cosmos.
Commonplace, yes - but also very peculiar. When changing from a liquid to a solid, almost everything in the universe contracts, growing denser. Water bucks this rule by expanding and becoming lighter. If water behaved "normally," icebergs wouldn't float, and the Titanic would not have sunk. Pipes wouldn't burst when the temperature drops below freezing, either.
But water's strangest and least appreciated characteristic is this: The two hydrogen atoms chemically bonded to one oxygen atom that comprise water are not linked in a straight line (180 degrees) but at an angle of 105 degrees. This fact alone has made life on Earth (and perhaps on endless other worlds) possible. The angle of 105 degrees gives the water molecule a kind of polarity, where the oxygen portion has a more negative attraction and the hydrogen portion a positive one. This makes water molecules align themselves in a network of weak connections, so that instead of being a loose mixture of individual molecules, water is a latticework that behaves like a much bigger structure.
This little feature changes everything. Without such bonding, room-temperature water would be like all other molecules of its size and weight: a gas. Its odd shape alone allows water to act as if it were a bigger molecule. It explains why your veins are filled with fluid instead of gas.
Water's process of freezing is curious, too. A full container of water is heaviest when it's about 40 degrees. Usually liquids are densest just before they freeze solid, because molecular motion is then slowest and crowding reaches its highest level. Well, the fact that water is densest some eight degrees above its freezing point is not only extremely unusual, it also keeps many lakes from ever freezing solid, allowing fish and other organisms to "winter over" safely near the bottom, even when the temperature stays far below the freezing point.
Here's why: When a still-mild autumn lake is cooled by freezing-cold air just above it, its surface is of course chilled first, from 60 degrees to 50 to 45...colder and colder. But this top layer cannot possibly reach the 32-degree freezing temperature without first passing through 40, and at 40 the surface layer is denser than the water below it (since 40-degree water is denser than water of every other temperature), so it sinks to the bottom. Now there is a new surface layer, but it too can't freeze without first reaching 40 degrees and sinking to the bottom.
In short, no freezing of the surface can take place until the entire lake's water, top to bottom, has reached 40 degrees. Only then can the surface chill to 39, 38, 37 - all of which are less dense than 40 and therefore capable of remaining at the top. Finally this top layer freezes and the lake's ice thickens through winter from that surface downward, sparing the marine life lurking safely at the bottom.
Since a lake's entire volume must be chilled to 40 degrees before any surface freezing can occur, large bodies of water never reach that situation, since water is a poor conductor of heat and cold. The world's major lakes thus have ice-free surfaces during most or even all of the winter.
Yes, living on a water planet is a strange affair.