When a word is repeated in a poem, it's usually for emphasis. And that's certainly the case with the repetition of the word "grey" in "Sea-Fever".

The speaker is so enamored of life at sea, so keen to take to the water once again, that it won't matter to him if the sea and sky are lonely or if there's a grey mist on the sea's face or a grey dawn breaking. The speaker just loves the sea; end of story. Irrespective of the weather conditions, he just wants to head back out to sea as soon as possible. In his repetition of the word "grey," the speaker emphasizes that it's not the beauty of the sea that matters to him so much as the amazing privilege of sailing upon it.