Random Riddle: My Grip

Often I must strive with wind and wave, and 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 puts me to sea.
They wish to capture what I must keep, but I can master them both if my grip holds out; if the rocks bring succor and lend me support and strength in the struggle.

What am I?

 

Random Riddle: My Grip