Not all poetry includes moral or ethical lessons. Sometimes, a poet is simply moved by an experience, an emotion, or a happening and feels as though the only way to capture it is to write it down poetically.
As for the existence of any moral or ethical lessons in Tennyson's poem "The Eagle," one could justify either through Reader-Response. Reader-Response is a critical approach where the reader's interpretation is of more importance than that of the author. In regards to the poem, one could easily justify that the poem is about taking the challenges of life on.
The eagle is perched above the "wrinkled sea." Observing the world from a place of solitude, the eagle falls "like a thunderbolt." One could easily state that the moral of the story is that regardless how tumultuous the world may seem, one must be willing to face the hazards in order to survive.
One can assume that the eagle sees something in the sea which he desires (like a fish). Even though the water beneath the eagle is churning, the eagle sees the only way to survive is to throw itself into the chaos of the sea. Metaphorically, one could look at the poem as being one which forces people to not give up when things look hazardous or frightening. Instead, like the eagle, mankind should be sure to take on the challenges in life head on. The parallel between mankind and the eagle is made more profound when Tennyson describes the eagle as having hands.