The English translation is awkward. Better sentences would be: "How much water is in the ocean?" or "What is the volume of water in the ocean?"
Berapa is a question word to ask about quantity (countable or uncountable). The translation in English is correct “what is ...” not “how much ...”
My sentence is being corrected to:
You used the wrong word. What's is the volume of water in the sea?
"What's is" is obviously a typing error.