If the piracy is happening on a large scale, it should be easy to find the unauthorized stream. If the piracy is happening on a small scale, then it's not a big problem for the stream source.
And we're both assuming that the people running those mirrors won't work out a way to disable the watermarks in real time. I've not read up on digital watermarking technology, but it's hard to imagine that it's bulletproof.
And yes, I'm assuming that robust-enough watermarking methodology is possible. I don't think it has to be bulletproof, I just think it has to be good enough to enable cancelling enough stream consumer accounts to deter casual mirror-ers. DRM isn't bulletproof either; watermarking is preferable for users because it doesn't require that the stream producer take control over your machine.