Yea, thou shalt be as he that lieth down in the midst of the sea, or as he that lieth upon the top of a mast.
Yea, thou shalt be as he that lieth down in the midst (the heart) of the sea - fancying himself secure as if he were in the midst of the land; implying the greatness of the danger; also the restlessness of mind and body, so that all things seem to swim around one, as if he were being tossed on the sea.
Or as he that lieth upon the top of a mast - where there is the greatest agitation.