Ultra Thread 140 is a versatile saltwater fly-tying thread.  It has the perfect amount of stretch to help tension down on your materials to help make for more durable flies.

The Ultra Thread 140 is becoming increasingly popular for saltwater flies. Its equivalent diameter in Danville for example is 6/0 a size most often used with trout flies. So you get a thin diameter with a breaking strength of around 2 pounds. Ultra Thread was designed for fly tying and isn't just a re-purposed thread from some other application. 

Perhaps bump up to the Ultra 210 (a heavier thread) if you're tying the biggest saltwater patterns.