Sea water or where it usually comes from (4,6)

I believe the answer is:

main source

I'm a little stuck... Click here to teach me more about this clue!

'where it usually comes from' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'sea water' is the wordplay.
'sea' becomes 'main' (synonyms).
'water' becomes 'source' (I can't justify this - if you can you should give a lot more credence to this answer).
'main'+'source'='MAIN SOURCE'

'or' is the link.

Can you help me to learn more?

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.