What has three feet, but no toes?

What goes up the chimney down, but can't go down the chimney up?

Oft I must strive with wind and wave, Battle them both when under the sea
I feel out the bottom, a foreign land. In lying still I am strong in the strife;
If I fail in that they are stronger than I, And wrenching me loose, soon put me to rout.
They wish to capture what I must keep. I can master them both if my grip holds out,
If the rocks bring succor and lend support, Strength in the struggle. Ask me my name!