The sentence "We must take care of the Earth's water so can us" is grammatically incorrect and doesn't quite make sense. It seems to be trying to express the idea that:

We need to protect Earth's water resources so that we (humans) can continue to thrive.

Here's a breakdown of the issues and a better way to say it:

* "So can us" is incorrect grammar. The correct phrasing is "so that we can..."

* "Us" is informal. "We" is more formal and grammatically correct in this context.

Here are some better ways to express the idea:

* "We must take care of the Earth's water so that we can continue to live on this planet."

* "Protecting Earth's water is essential for our survival."

* "We have a responsibility to conserve water for future generations."

The message is important: water is a precious resource that we must protect.