Riddle Fans

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 master them both if my grip holds out.
If the rocks bring succor and lend support, strength in the struggle. Ask me my name!