The speaker is a girl who can't bear to see herself. Water is everywhere in the poem, in rivers, lakes, seas, even ponds and small dams, and each one works like a mirror that "taunts" her. She keeps away from it, staying up on a hill and watching from a distance. The title, "It is I," hints at what she's hiding from: her own face.
The poem also suggests a change. She was once "a happy girl within the waves," and now she sits on the sand, cut off from that joy. The sea, a place she used to belong, has become something she fears. The lines about beauty being "skin deep" and being "far from a ten" show where the pain comes from: she judges her looks harshly and won't pretend otherwise.
The mood is lonely and heavy, with a quiet resignation. The ending repeats the hill and the sand, so she seems stuck in the same spot, alone and unwilling to look.