The terms "water" and "poisonous" you got there are ambiguous.

If you speak about the pure water (ddH20 without RNAse and DNAse), it's already poisonous to some living creatures under certain conditions/circumstances.

It's an interesting question, and one tackled a few years ago by a three of the world's greatest scientific minds.

They compiled their findings here.

We would probably just function like Ewoks.