Here's the poem:
Night Thoughts Afloat
By bent grasses
in a gentle wind
Under straight mast
I’m alone tonight,
And the stars hang
above the broad plain
But moon’s afloat
in this Great River:
Oh, where’s my name
among the poets?
“Retired for ill-health.”
what am I more than
A single gull
between sky and earth?
Much of Tu Fu's poetry deals with with levels of time and place: man's aloneness in the past, present, and future and in space, air, water--even in memory.
In other ancient Chinese poetry (Du Fu's "The Broken Boat"), the boat is a symbol of homelessness. So says Enotes editor:
The boat can indeed be regarded as a thematic symbol of this homelessness.
Here, the speaker compares himself to the boat, a gull, a name, and the moon, all lonely creatures surrounded by void. So there are four levels of loneliness and homelessness: the moon is lonely in space; the gull is lonely in sky; the name is lonely in memory; and he is lonely on water. No one or nothing is on land, at home. Only his thoughts (the poem itself), however, can permeate and connect all levels.
First, the title "Night Thoughts Afloat" combines the speaker with boat imagery: both he (in the boat) and his thoughts are "drifting, drifting."
Next, the speaker says "the moon's afloat" like his thoughts (title) and the boat from which he narrates.
Next, the speaker wonders where his "name" fits in the pantheon of poets: this is a metaphysical analogy. This metaphor has no physical space; rather, it is an intellectual construct, and it is bound by time: how do I (in the present) compare to the past?
Finally, the speaker directly compares himself to "a gull between earth and sky."
This poem is very meta- (meta-cognitive and meta-poetic). It's thinking about thinking; poetry about poetry. Thoughts are the key. His thoughts can be lonely in space, air, water, memory, past, present, and future.
Maybe the thoughts have found a home on paper...
I think that the poet is comparing himself to at least a couple of different things here.
You can argue that he is comparing himself to a boat in the first stanza. However, he may just be saying that he is floating along in the boat.
He is certainly comparing himself to a sea gull in the last stanza. He is saying that he is no more important or lasting than a bird that just drifts along in the air. Neither he nor the bird is having much of an impact on anything down on earth.