Crucial to understanding this poem as a whole is realising that Sylvia Plath is examining ageing through a very interesting medium. By telling the poem through the persona of the mirror, she allows us to see the woman who comes to the mirror and despairs over her ever-increasing wrinkles and crows-feet.
You are completely right in identifying that in the second stanza, the mirror suddenly describes itself using a metaphor, saying that it is now "a lake." It is not literally true, it is just using this comparison to describe how the woman reacts to it and to prepare us for the shocking ending of the poem. Let us examine this second stanza in detail:
Now I am a lake. A woman bends over me,
Searching my reaches for what she really is.
Then she turns to those liars, the candles or the moon.
I see her back, and reflect it faithfully.
She rewards me with tears and an agitation of hands.
I am important to her. She comes and goes.
Each morning it is her face that replaces the darkness.
In me she has drowned a young girl, and in me an old woman
Rises toward her day after day, like a terrible fish.
Describing herself as a lake allows the metaphor to effectively describe the action of the woman in her obsession with the mirror and how she spends so much time looking at it, trying to find who "she really is." As the poem says, the mirror is "important to her." However, as the last two lines of the poem comment, the mirror is so important to her that effectively the woman has "drowned a young girl" in the "lake" that the mirror represents, and in the mirror each day an old woman rises towards her. Thus the poem comments upon the finality of ageing and its inevitability. The horrid simile that is used to describe this process, "like a terrible fish," really gives the poem a lot of impact.
So, the mirror describes itself as a lake to allow us to think about the ageing process in different ways. In a sense, it is a kind of extended metaphor that runs throughout the second stanza and prepares us for its shocking finale.