What border did He make for the water?
Rashi: It is sand around the edge [of the sea].
Why does it say "Bal Yeshuvun Lechasos ha'Aretz"?
Radak: They should not cover the land like initially. This is from Hashem's ability, Chachmah and mercy on His creations. When the water gathered in one place, it was very high above the land. It was spread over the entire land. It gathered in one place and stood in a heap - "Kones ka'Ned Mei ha'Yam" (33:7). Water's nature is to flow from high to low; Hashem withheld them from this nature, that the sea waves will go until the shore, and not pass the border - the sand on the sea shore. It says "Yordei ha'Yam" (107:23 - they descend to the sea) because the shore is higher than the sea.
