80% water. Breakdown: 75% ocean, 5% freshwater. Further breakdown-- that 5% is mostly glaciers and ice. The temperature sits solidly between 50-70 degrees Fahrenheit in the temperate regions. In the arctic, sometimes it doesn't get above 40 in the summer.
How are people supposed to move around? Obviously with ships.
Another question: what's in the water? Nothing. Absolutely nothing lives under the waves. You're perfectly safe. The forests of kelp hold no sharks grown to obscene sizes. The deep black water of the arctic is not filled with unknown monsters waiting to drag ships to the bottom. When you look into the water, and your reflection is just a bit off, blinking independently of your own eyes, that's just a trick of the light.
There are no creatures just shy of human, or just past human, below the surface.
It'll be fine.