In the context of a poem, an apostrophe is an address to an absent person, thing, or idea. In Byron's "Apostrophe to the Ocean," the speaker is obviously addressing an ocean.

In true Romantic fashion, the speaker soon makes it clear that he loves this particular aspect of nature. What he loves about it most of all is that it is completely free of man's control. No matter how powerful man may become, no matter how much of the globe he may explore and exploit, he will never ever be able to control the vast oceans of the earth.

The speaker cannot help but admire the destructive power of the "deep and dark blue Ocean." The ocean isn't just beautiful; it is sublime in that it exercises an awesome power over us. And try as we might, this power cannot be resisted. The ocean will move to its own unique rhythms, and there's absolutely nothing that we can do about it.

Man's helplessness in the face of nature is symbolized by the numerous shipwrecks for which the ocean is responsible:

The wrecks are all thy deed ...

The speaker is almost awestruck as he describes the destructive power of the ocean. He clearly admires the ease with which it wrecks ships and kills the men who serve aboard them.

For a Romantic like Byron, it makes a refreshing change to see a part of nature that does not and will not yield to man's designs. In the age-old battle between man and the sea, there can be only one winner, and it's certainly not man.