Psalms 96:11

Let the heavens rejoice, and let the earth be glad - Let all worlds be full of joy, as they are all interested in the fact here stated. The universe is one. It has been made by the same hand; it is under the control of the same mind; it is governed by the same laws. The God who reigns on earth reigns in heaven; and what affects one part of the universe affects all. Hence, in all the manifestation of the character of God, whether made in heaven or in the earth, it is proper to call on all the universe to partake in the general joy.

Let the sea roar - In praise to God. It is not uncommon in the Scriptures to call on inanimate things to praise God. Compare Psalms 148:7. The same thing is common in all poetry.

And the fulness thereof - Its abundance. That which fills it. All that it contains. That is, Let all that dwell in the seas praise God. His reign is an occasion for universal gladness. All in the inanimate world; all among the irrational tribes of being; all in the air, in the waters, or on the earth, have occasion for praise, and would render praise if they could appreciate the wisdom and goodness evinced in their creation. Though unconscious, the lower creatures seem to celebrate his praise; but man only can give an intelligent utterance to thanksgiving.