Right now I'm just using nylon thread, but it seems limited to extremely light things like paper or cards. What type of thread should I be using if I was thinking of floating something like a coke can or possibly a bottle. I'm sometimes struggling to float a pen without the thread breaking. How strong is kevlar thread and is it as cheap and easy to obtain as nylon?

Kevlar thread is stronger than the regular invisible thread but not strong enough to float a bottle. I doubt if it can float a Coke can either. However, I have never tried it. But I floated stuff lighter than a Coke can and the kevlar thread ripped depending what you do.