All teas came from China.
茶 is the kanji word for "Cha" or Tea.
If you want a tea, you say "Ocha o Kudasai."
Water is 水 mizu.
If you want a whter, you can say "Omizu o Kudasai."
What is O before Cha and Mizu?
By having O, you are more polite.
You never say "Cha o Kudasai."
Although, you can say "Mizu o Kudasai."