Remember that M. Night Shyamalan movie Signs where (Spoiler Alert) it turns out the aliens biggest weakness is water?  While I totally think it's silly for those aliens to try to invade a world that's made up of 71% water, we don't know for sure that water is required for life.  We think it is because it's basically true for all the life we know about, but it is possible that life could have evolved somewhere in the universe without it.  

(As a side observation, the Wicked Witch of the West shares the weakness of the aliens from Signs.  Is she one of them???)

I think it would be funny if there were some aliens who didn't even know what water was and landed their spacecraft somewhere in the middle of the ocean, assuming it was a nice flat place to set down.  Then when they opened their hatch and their ship filled with water and sank they'd be in for quite the surprise, I'd say.