While the Earth is often described as round, it is more accurate to say that the Earth is an oblate spheroid. This means that its shape is slightly flattened at the poles and bulging at the equator due to its rotation.
As for the ocean appearing flat, it is an optical illusion caused by the vast scale of the Earth's surface compared to the curvature of the planet. When we observe the ocean from a typical human-scale perspective, such as standing on a shoreline, the curvature of the Earth is not readily apparent.
In reality, the Earth's surface, including the ocean, follows the curvature of the planet. If you were to travel over a large body of water, such as crossing an ocean, you would gradually notice the curvature as you move over its surface. Similarly, when viewing the Earth from space, it becomes evident that the planet is not flat but has a curved surface.
The reason the ocean's surface appears flat to our eyes is because the Earth is very large compared to our immediate surroundings, and the curvature is not easily noticeable over short distances. However, over larger distances or from a global perspective, the Earth's curvature becomes evident.