A seemingly uncomplicated, easy to read poem transforms itself unexpectedly into an almost philosophical poem about life or losing life and how it just fits into the whole scheme of things. Nobody stops and considers or "look(s) long enough" and reflects upon the effects of unforeseen and often tragic events.
The lake perhaps signifies all the complexities of life. A serene and peaceful place can be altered in an instant into a place of despair - much like the events of someone's life. An apparent average day can change someone's future or rapidly end it!
At first the lake is "in the background" and before the reader has even had a chance to consider this lake with the "low hills" beyond it, we are thrown into turmoil. The lake becomes central to the poem and the place that is responsible for the speaker's death.
The speaker is only now coming to terms with what happened to her "some time ago" and the lake also possibly signifies how, for others, life goes on. She is talking about the photograph as if she is alive and the sharp contrast "after I drowned" when the reader must deal with her pain is seen in the lake where "the effects of water/ on light" distort the reality. Who would have thought that such a beautiful, serene place could inflict such devastation? The peace has already returned but the reality will always be there "just under the surface."