Off 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, but I can master them both if my grip holds out and if the rocks bring succor and lend support, strength in the struggle.
What am I?