The sea looks "wrinkled" like a shirt that needs ironing. We can't help thinking of a sheet of tin foil that has been crumpled up and then spread out again.
The wrinkles on the sea are waves seen from a very great distance – the eagle's perspective. The waves are so small compared to the huge body of water that they "crawl" slowly toward shore.
The eagle looks down on the world from a privileged position. He doesn't see the tumult and chaos of crashing waves; he sees only small lines on the water.
The words "wrinkled" and "crawls" normally relate to people. Recall the first line of the poem, where the eagle's claws are compared to "crooked hands." What kind of person would have crooked hands? Sounds like an older person whose body shows the signs of advanced age. Well, here too you have a physical indication of old age in the image of wrinkles. But suddenly you also have the image of an action performed by babies: crawling.
Tennyson very subtly uses words that make us think of human life cycles. Nature carries these cycles within it; or, more accurately, the speaker projects them onto nature. The speaker thinks of the natural world as a body undergoing continuous decay and renewal.
He watches from his mountain walls,
The eagle performs the passive action of watching along with all his standing and grasping. If the eagle had written a journal entry for his day so far, it might read, "Today I stood on the cliff…held on to the rock…looked around…thought about letting go of the rock, but then how would I stand? So I kept holding on…stood…and looked around."
We don't know what the eagle is looking for, or at. Our guess would be food. Most people know that eagles have fantastic vision – much better than ours – and that they like to perch in high places so they can survey a large area for animals to munch on. This eagle might be thinking about grabbing himself a tasty fish out of the water.
The mountain walls are described as "his," as if he owns them. Not that anyone else could even reach the walls to claim them. The walls are the eagle's turf.
And like a thunderbolt he falls.
Finally, all our careful observation of the eagle pays off. In a magnificent show of grace and speed, he dives off the cliff and shoots downward in a straight line. Maybe he's seen something to eat or else just wants to stretch his wings.
Interestingly, the verb used to describe the eagle's flight is also passive: "he falls." That's a lot different than "dives," "swoops," "soars," or any number of more active verbs used to describe flight. The eagle lets gravity do all the work.
A thunderbolt is a good simile to describe how the speaker views the eagle. The speaker only catches the eagle zooming down towards the sea, and not the moment he leaps off the cliff. When you look for lightning in the sky, you can never predict when there will be a flash, and your eye can only move fast enough to catch a split-second of bright light.
The last two words of the poem echo the last two words of the first stanza. Both stanzas end with a subject and verb that come after the descriptive part of a sentence. The poem has been building to this climax from the beginning.