Praying for you it's not the head mate - is there noticeable signs of water on the engine when you've stopped (did you check?). Was there water leaking from anywhere noticeable (I know that sounds stupid, coz you'd already know I guess where it's coming from, but I'm just asking the obvious).

If not, it could be anything from a leaking pipe to a cracked head and anywhere in between, but either way, you need to suss it out quick.

Good luck with it mate, and hopefully others will have input also.