What would happen if...

you take the water?

If you took the water away, the Sea Slug would look something like the picture below. That is, until it's outer layer of mucus dried up. Then it would die.

you touched it?

You'd be in for a shocking surprise. The Portuguese Man of War jellyfish is a favorite treat of it. They take the Man Of War's toxins and amplify them for their own use.

there were no more jellyfish?

It simply wouldn't sting you anymore. However, because of its small size, it would quickly be eaten without its toxic defense strategy.