The events of Water World take place in the future, after the polar ice caps have melted and the earth is completely submerged in water. People in this post-apocalyptic society will have to adapt to life on water, with floating towns and an economy based on marine resources, which makes for a very grungy and disheveled appearance for most Water Worlders.
There’s a lot of room for innovation with this costume as there is no single character that really stands out. Get some dirty old used clothes, shred them up and soak them in dirty to capture that well-worn post-apocalyptic appearance that will amaze fellow cosplayers and party goers.