Early on, the narrating speaker tells us that this strange new place is a "[l]and of streams" (10). So, it's probably not that surprising that streams come up over and over again. The speakers in this poem seem kind of in awe of the landscape they are describing, and they take special care to give us a sense of the beauty of the streams in the land of the Lotos-eaters.
Line 8: In this line, the narrator compares the "slender stream" (alliteration alert!) to a "downward smoke." That kind of comparison, where you use the word "like" is called a simile. The phrase "downward smoke" is also an oxymoron, since smoke floats up, not down. It's just a little hint that things aren't quite how they should be in this new land.
Lines 10-13: This is a long description of the beautiful streams and waterfalls in the land of the Lotos-eaters. Tennyson uses similes to compare the stream to smoke or thin fabric ("lawn"). He also throws in some S word alliteration ("slumberous sheet").
Line 55: We circle back to the streams again here—they're clearly a really important part of this strange new landscape. In this case we hear a little more about the flowers that "weep" into the stream. That's another great example of personification, and it adds to the sense that everything in this place is filled with thoughts and feelings.
Lines 99-100: Here the sailors dream about lying around and listening to the stream all the time. Notice how Tennyson breaks that thought up over two lines? We call that little poetic trick enjambment.