@vzm i agree, and I'm going to prove you right
Water is ~800 times more dense than air. This density provides buoyancy, which means fish don't have to counteract the forces of gravity. This leads to something called indeterminate growth, which means as long as you're living and eating, you keep growing. This is why we have a lot of species of fish that can grow to quite literal monstrous lengths and sizes.
Water is in-compressible, which means it must be completely displaced to move through it. Due to something called the Reynold's Number [R] (which is a ratio of inertial forces to frictional forces as they relate to solids and water movement) your size in the ocean determines how well you can travel. If you are small, water is very hard to move through (R<1, frictional forces dominate, water basically feels like molasses); if you are large, water is easier to move through (R>200, inertial forces dominate, water basically feels like alcohol). This is why large species have very wide ranges of habitat.
Water being very dense and in-compressible has other passive properties. Sound travels through water better than air; so well in fact that there is a magic-like depth called the sofar channel where sound can literally go on forever. Whales uses this to communicate and find each other. These properties also helped fish develop suction feeding/vacuum effect, which is one of the primary forces driving the diversity of fish.
Water is a universal solvent, and sea water is one of the most corrosive things on the planet. Being a solvent means it can hold dissolved gases like oxygen, carbon dioxide, and ammonium (the ionized, nontoxic form of ammonia, which is very toxic). This means fish can safely eliminate waste products across their gills. PS this is also how our blood works with the help of hemoglobin. So thanks water.
Light is heavily absorbed by water, which is known as electromagnetic absorption. Because light is made up of differing wave lengths (with red being the least energetic and violet being the most energetic), you lose colors the deeper you go in the ocean. This means that most deep sea fishes are "color blind" and only sea in shades of green/blue. This provides one species of fish in particular the ultimate evolutionary cheat code: the Red Dragon fish not only kept the ability to see the color red, it also emits red light using an organ under their eyes like a spot light. This means they can use the light to hunt fish without being seen by predators themselves. Hollywood wishes it could make shit like this up.
Because water is so heavy, water pressure increases rapidly with depth. Every ~10 meters (or 33 feet) of depth you have the equivalent of 1 additional weight of Earth's atmosphere (atm) exerted on your body. This has an effect on dissolved gases such as Boyle's law (the volume of a gas is inversely proportional to the pressure; in layman's terms, the volume of a gas increases as pressure decreases), which in turn has a effect on the amount of dissolved gases can be available in water. Increased pressure also effects the structure of proteins, which is why most deep sea fish that are brought to the surface look vastly different when they are at depth. This also contributed to the evolution of something called chaperone proteins/molecules, which are proteins that help normal proteins keep there shape in vertical migratory fishes like lantern fish.
This has been a short ted talk on why water is awesome
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