If the surface of water always levels itself out and is always flat. How is it possible for it to span “around” the earth in the oceans?
It isn’t flat. It always conforms to a spherical section with the center of the Earth as its center. The same is true for a pot of water on your stove or an ocean.
Because Earth’s gravitational pull is what keeps water from flying off the earth ,and earth basically has an equal pull around its sphere, water is pulled to the lowest(or closest to the center of the globe) elevation. Therefore filling, for the most part the lowesat areas on earth and conforming to the earth itself. Some exceptions apply such as the effect of the moons gravitational pull on our oceans which cause tidal movements and surges toward the direction of the moon.
Also you may notice a curve of another kind when you poor a small amount of water on a flat surface. Notice the slightly curved edges of the water caused by cohesion. Cohesion is the desire of water to pull together, like when you put two drops of water real close together on a plate of glass and they magically move into each other to form one.