The creation of the world and the mastery over the forces of nature stand at the center of contemplating God's greatness. By their very nature, waters were meant to flood and cover the entire surface of the earth. However, to allow dry land to emerge and life to exist, God gathered and concentrated the oceans into defined spaces. The primary approach among commentators is that this describes the initial miracle of Creation. Without this act of gathering the waters into one place, no living creatures other than fish could survive [רד״ק, מאירי, אבן עזרא].
Beyond the initial act of creation, a continuous, wondrous phenomenon takes place. The ocean waters are actually higher than the dry land, standing like a towering wall or a piled up mound of earth [רש״י, רד״ק, מצודת ציון, ביאור שטיינזלץ]. Yet, they are held back at the shoreline, never spilling over to flood the world [רד״ק, אלשיך, מצודת דוד]. While this might recall the dramatic splitting of the Red Sea, its purpose here is to demonstrate God's constant, everyday supervision over nature [אלשיך]. Furthermore, vast amounts of deep, underground waters are locked away in special storehouses [רש״י, מצודת ציון, ביאור שטיינזלץ]. God sets firm boundaries for these depths so they do not break forth and destroy the earth, as they once did during the generation of Enosh [אלשיך, מצודת דוד, מאירי, ביאור שטיינזלץ]. Some sages also connect these deep waters to the process of rainfall at the dawn of creation, where the underground depths caused the heavenly storehouses of water to fill [תורה תמימה].
Within the broader structure of the psalm, the mention of the sea completes the description of the visible parts of creation, following the earlier mentions of heaven and earth [אבן עזרא]. Some commentators suggest that this also completes the representation of the four classical elements, with fire and air hinted at previously, and water and earth emphasized here [רד״ק]. Yet, the awesome power of God controlling the seas often stands in painful contrast to the lowly actions of humanity. The Talmudic sages relate that Rabbi Yonatan would weep when reading of this immense power, mourning the fact that against such a mighty God, King Ahaz dared to rebel, abolish the study of Torah, and create a small, narrow idol for worship [תורה תמימה].