What's a flexible, strong string or yarn I can use in this toy? I made this flexible geometry toy based on one I saw in a learning store. You can bend it into different shapes, and it's fun.

Unfortunately, all I had on hand was some yarn to make the inside support. 

I'm concerned that the yarn is eventually going to snap, or the knot is going to unravel. It's also not water-proof, so it might not be that great for extended kid use due to the messes they make.

What can I use instead?

I'd like something that: 


Is strong, so it doesn't snap
Has some flex, so the toy can bend like it's supposed to
Is somewhat waterproof
Can easily be knotted in a way that won't unravel
Be able to have at least 3 strands of it threaded through a 4mm wide straw


Here's the toy, to give you an idea of my requirements:
Instead of yarn or thread, would fishing line work for you?


Different strengths available.
Is flexible.
Is made to be in water.
Can be knotted.
Variety of diameters available.