Most people at this early stage would surely assume that whatever this infection/outbreak is, it will ultimately (hopefully) be dealt with and cured. Its unlikely that they'd be thinking... "It's definitely the end of the world so let's start killing people."
Add to that, the fact that they're also at sea and therefore, pretty damn safe and this makes the idea that they would elect to abandon civilisation and become bad-ass pirate muderers, a little hard to swallow.
I'm willing to suspend disbelief but its definitely a legitimate criticism.
Maybe, but I can also see the argument that this is a very different and MUCH scarier kind of outbreak than the kinds we're used to in our world. People aren't just spreading disease and dying, they're coming back from the dead, chasing people around in giant hordes and then freakin eating them.
Add to that the fact that the world's richest and most powerful governments have seemingly collapsed almost overnight, and I think people have cause to freak out and overreact a bit.