What does the speaker need when he is at sea in "Sea Fever"?

"Sea Fever" is a poem by John Masefield. Throughout the poem, the speaker lists the things for which he "asks" when he is at sea. These things are basic at first—a tall ship and a star by which to navigate—but become increasingly descriptive as the poem goes on. As we trace what the speaker asks for, we ourselves fall under Masefield's spell as we, too, experience the wanderlust that makes the speaker feel that he "must go down to the seas again."
Here is the first stanza, in which I have bolded what the speaker "needs":

I must go down to the seas again, to the lonely sea and the sky, And all I ask is a tall ship and a star to steer her by And the wheel's kick and the wind's song and the white sail's shaking, And a grey mist on the sea's face, and a grey dawn breaking.

The ocean is personified in this stanza: she has a face. Thus, the speaker reminds us that the sea is not an inanimate thing but can interact with us. Note too the use of alliteration in this stanza: the repeated "s" sounds in the second and third lines and the "w" sounds in the third line create that sweeping sound that we often associate with the ocean and the rolling waves.
Now to the second stanza. Again, I have bolded what the speaker "needs":

I must go down to the seas again, for the call of the running tide Is a wild call and a clear call that may not be denied; And all I ask is a windy day with the white clouds flying, And the flung spray and the blown spume, and the sea-gulls crying.

In this stanza, all five senses are conjured: we hear the seagulls, we see the white clouds, we smell and taste the saltiness of the flung spray, and we feel the wind. No longer are we merely reading the poem; we are on a ship on the high seas, feeling every sensation that this speaker longs to feel.
Here's the last stanza:

I must go down to the seas again, to the vagrant gypsy life, To the gull's way and the whale's way where the wind's like a whetted knife; And all I ask is a merry yarn from a laughing fellow-rover, And quiet sleep and a sweet dream when the long trick's over.

Here we see mention of another person, a "fellow rover," who tells an enjoyable story to the speaker. The speaker does not romanticize the ocean, however. The he describes the wind like a "whetted knife" suggests that there is discomfort and even pain in the sailor's life.
That last line is where Masefield wants us to really think about what the speaker is asking for. The quiet sleep and sweet dream could of course be read literally: a night's sleep with pleasant dreams after a "trick," which in sailing lingo means a watch (a shift of four hours watching, eight hours resting). However, because this is the last line of a poem, we are invited to think more metaphorically. It could be that the "long trick" means a life and the "quiet sleep" refers to death. So, after a life of adventuring and exploring and experiencing, this speaker will be content with a peaceful death.
In essence, the speaker needs to live a vibrant and exciting life at sea and to live it fully in order to be content when his life comes to a close.