A person sits outside as waves approach, and instead of running, they stay put. Everyone else is crying with fear, but the speaker is calm, even gently addressing someone as "my dear." The waves are probably more than water. Judging by the imagery, they stand for whatever trouble life throws at us, and the poem is about choosing how to meet it.
In the second stanza the waiting continues, and then the waves finally overtake the sitter. Strangely, there's a smile. The line "the lesser of 2 evils" suggests that facing the wave is better than running from it, or than living in dread of it. Once the wave arrives, the fear disappears: "No fear no more." The closing image of waves "knocking at the door" feels almost polite, as if trouble has become something to answer rather than escape. The mood is quiet, steady and a little defiant, with acceptance that feels like a kind of strength.