On the literal level, the poem is simply a description of an eagle who sits high on a rock above the water, then dives down. As a poem, it gains the power it has through the quality of its description, its structure and word choices, and what non-literal things it alludes to.
Tennyson personifies the eagle, making it "he" and more active and intentional than it might be. The rhyme and rhythm are markedly present. References like the eagle's presence near the sun links it to Icarus in Greek mythology, making it sound like an older human filling that position of mythic youth.