Were the Zebra Mussels transported here on purpose or not?

That's pretty easy information to find. Here's a quote from Wikipedia- "The species was originally native to the lakes of southern Russia and Ukraine, but has been accidentally introduced to numerous other areas and has become an invasive species in many countries worldwide"

No- ballast water is just taken up and dumped when no longer needed.   Now it's changed out mid ocean so most foreign wildlife is flushed out.