Originally Posted by jackdale
If it floats, it might make a good stern line.
If stretches it is useless as a sheet or halyard.
Climbing rope is all designed to stretch in order to absorb dynamic loads that would otherwise be trasferred to the body.
If you use ascenders to climb your mast, you could haul a climbing rope up on your halyard and climb that rather than impose the wear and tear on the halyard itself, though with 3 months use in a climbing gym, much of the ropes intial strength will be gone. Most climbing ropes are intended to be retired after 10 or so falls.
As others have said it could be handy for backup mooring lines or a long emergency tow line, but it really wouldn't be much good for running rigging, except maybe for a preventer or boom brake system, where its properties would help absorb the energy of an accidental gybe.