The theme of "Once by the Ocean" is the inevitable passing of time and its slow destruction. There is a heavily foreboding tone established at the outset of the piece. The water and waves "shatter" into the shore and "The clouds were low and hairy in the skies,/ Like locks blown forward in the gleam of eyes." I picture a very gray and stormy sky.
The waves are plotting against the shore as well. This personification of the ocean furthers the sense of foreboding.
Great waves looked over others coming in,
And thought of doing something to the shore
That water never did to land before.
The reason this is foreboding is that the speaker has the "feeling" that "The shore was lucky in being backed by cliff."
The cliffs bring us even more evidence towards the destruction of the passing of time. The cliffs will only last so long against the constant battering of the waves. Erosion will eventually take over and bring the cliffs down into the water.
Finally, a sense of destruction at the hand of God is offered in the last lines:
It looked as if a night of dark intent
Was coming, and not only a night, an age.
Someone had better be prepared for rage.
There would be more than ocean-water broken
Before God's last Put out the light was spoken.
God's plan dictates that everything in nature must grow old and die. Frost gives this poem its final message by including God as the initiator of the destruction.