Coleridge's poetics, at least in one sense, mirrors the poetics of other romantic poets in that it emphasizes the perceiver as the subject of poetry. "Mariner" and "Kubla Khan" reflect this.
Coleridge's goal was to write a poem of perfect imagination. "Mariner" is a poem of the imagination, and "Khan" has as its main subject the imagination. The emphasis in both is on the perceiver, the speaker, although "Mariner" does feature respect for nature as its theme.
And imagination is of the speaker. The romantics believed that imagination was something that could and should be cultivated. Keats worries that he will die before his imagination is cultivated in "When I Have Fears That I May Cease To Be," for instance, and Wordsworth describes how nature changed him in the five years after he first experienced it in "Tintern Abbey." Coleridge is exercising his imagination in the two poems you ask about.
Again, the focus on the imagination of the speaker amounts to a focus on the perceiver. The objects on which the speakers focus on, are not nearly as important in the works as are the poets, or perceivers, themselves. This constitutes Coleridge's poetics.