It's not about having anchors, it's about not being affected by the top layer of water which moves up and down due to waves. If you submerge (your center of gravity) just a few meters under the surface, you'll suddenly be much more stable and much less affected by the waves. Oil rigs, just like ice bergs, have a large part that is submerged and invisible on the surface. Look at this for example: https://s-media-cache-ak0.pinimg.com/736x/73/64/d6/7364d6da3...