In "Sea Fever," the speaker describes how much he misses the sea and how he would love to return to it. Interestingly, this poem is often seen as autobiographical, as John Masefield did in fact spend some time at sea himself. This is what makes the reader really relate to this poem, as it becomes clear that the speaker has a sincere and authentic longing for the sea.
The first thing that the speaker is missing is the life onboard a ship. He misses all the little things that are associated with being at sea, the things a sailor would experience every day. For example, the speaker tells the reader that he misses "the wheel's kick," "the wind's song," and "the white sail's shaking."
Secondly, the author misses the physical sensations of the rough nature of the sea. He tells us that he wants "a windy day with the white clouds flying," a clear indication of the natural forces that one experiences when being at sea. He longs for witnessing the sheer force of nature, which can be experienced so powerfully when at sea.
Lastly, the speaker misses the sense of freedom that being at sea brings. He tells us that he longs for the "vagrant gypsy life," a life where he can simply move from one place to another without much care in the world. When on a boat, you can simply set sail and leave one place for another whenever you want. This freedom is what the speaker is clearly missing.