About 71 percent of the surface of the Earth is water-covered.
The natural physics of water is to seek and find its level.
Rivers don't run uphill, and water doesn't wrap around a ball.
See? Level.
See also: https://www.youtube.com/results?search_query=%22flat+earth%22
The global earth is gigantic. It is so large when compared with a person that in a location right nest to the person, the sea seems level. Actually, there are often waves which have peaks and troughs. The sea isn't level at all, even on a tiny, local section of it.
The term "sea level" is a misnomer the way it is being used by flat earth people.