Let us remind ourselves that similes are comparisons established between one thing and another indicated by the word "like" or "as." They are very different from metaphors, which likewise compare one thing with another but they assert the comparison directly without the need for "like" or "as." If we view the poem with this in mind, we see that the poem does not use any similes at all until Part III and the arrival of Sir Launcelot on the scene. Note the way that similes are used to describe his appearance and imbue him with a vitality and life that heightens the contrast to the dull existence of the Lady of Shallot:
The gemmy bridle glittered free,
Like to some branch of stars we see
Hung in the golden Galaxy.
Likewise we are told in the next stanza that:
The helmet and the helmet feather
Burned like one burning flame together
The focus on comparing him to a flame and then to stars glittening at night combine to make him attractive and distinct, full of life and vibrant. His appearance is then compared to a meteor in another simile:
As often through the purple night,
Below the starry clusters bright,
Some bearded meteor, trailing light,
Moves over still Shallot.
Lastly, a simile is used to describe the Lady of Shallot herself in Part IV of the poem and her appearance as she looks down to Camelot in her boat:
And down the river's dim expanse
Like some bold seer in a trance,
Seeing all his own mischance--
With a glassy countenance
Did she look to Camelot.
This simile indicates the way in which the Lady of Shallot is contemplating her own doom, now that she has made the curse come upon herself through her actions and embraced life.