Dyneema

Steel, but it floats

Dyneema (UHMWPE) is twelve times stronger than steel by weight, floats on water, and barely stretches. It's the cordage of racing yachts and ultralight hikers.

The catch

strength   12× steel, by weight
stretch    ~3% — very static
float      yes
cost       high; knots slip (use splices)

Best for

Standing rigging, winch lines, ultralight gear. Knots tend to slip on it — learn to splice, or use a proper friction hitch.