On the far side of the moon which you can see from Gensokyo, there is a sea with a lot of water. The light reflected from the Earth on the water's surface forms a kind of boundary. If you want to travel from the Earth to the moon, you have to cross this boundary.

The Sea of Tranquility has few waves. It is possible to see the Earth in the daytime. Nothing lives in the sea.

Peach trees grow along the coast. Perhaps this is part of Toyohime's secret walking course. When she gets hungry on her stroll, she can eat some peaches... She could probably stroll like this forever.

When Reimu and some others came to the moon on a rocket, they crashed into this sea. They fought with Yorihime here, but they were no match for her at all.

If the sea is a boundary, then Toyohime and Yorihime must cross this sea when they travel to the Earth... It is kind of cute to imagine the two of them showing up completely soaked at Eientei. Eirin might lend them towels to wipe themselves down.