(rock climbing) A climb without safety equipment over water deep enough to provide safety after a fall.

(rock climbing) To climb without safety equipment over water deep enough to provide safety after a fall.

(military, nautical) A naval force capable of independent military operations on the open sea with a global reach.

plural of blue-water navy

Something transient or ephemeral. / An ill-defined or variable border between concepts or things.

plural of line drawn on water

A mark, such as a line of seaweed, showing the lowest level reached by a body of water. / (by extension) The nadir or lowest point of something.

plural of low-water mark

(idiomatic) try to achieve a goal despite huge resistance, being an uphill battle

(simile) Unquestionably dead.