The Moon’s craters are often called seas. Here’s an excellent visualization by Fummy of how those dark shapes on the moon’s surface connect together into one big lunar ocean.
The lunar seas are actually impact craters formed about 3.800 million years ago when some enormous asteroids fell on the Moon’s surface. They were then filled with lava which cooled down and solidified, creating these curious seas.
Here’s what they look like without the water added:
And here is what the Moon would look like if it had both water and an Earth-like atmosphere: