Psalm 33:7

"He gathereth the waters of the sea together as an heap,....Which was done on the third day of the creation, by means of which the dry land appeared, Genesis 1:9; when the waters of the sea were piled up as an heap, and stood higher than the earth, as they now do; and which is a wonderful instance of the power and providence of God, to bound them, and preserve the earth from being overflowed by them, Job 38:9; he layeth up the depth in storehouses; that is, large quantities of water, for which he has his treasure houses, as for the wind, hail and snow, Psalms 135:7; and these are the clouds of heaven above, and the fountains of the great deep below, which the Lord opens and stops at his pleasure; see Genesis 7:11."

- John Gill's Bible commentary on Psalms 33:7

 

As KenH pointed out, it is a reference to the creation. The psalmist is urging folks to praise God with all their heart and recounts the great and marvelous things God accomplished to warrant such praise.

peace to you

 

Genesis 1:7-9, ". . . divided the waters which were under the firmament . . . And God said, Let the waters under the heaven be gathered together unto one place, and let the dry land appear: and it was so. . . ."
Psalms 33:7, ". . . He gathereth the waters of the sea together as an heap: he layeth up the depth in storehouses. . . ."

 

I think the literal idea is God confined the waters to set boundaries, as in putting liquid into a container. And as pointed out above, God's power of creation is in view.

 

I think it a redundant reference to containing the water.

 

At the flood, not only did it rain, but the “fountains of the great deep” came up, Genesis 7:11.

The storehouses are the underground aquifers where water has seeped into the ground.

peace to you