No you cannot complain now. It's too late, and it sounds like you didn't even object.
There's no such thing as a "water break."
He could have taken an injury time-out (5 minutes for unofficiated matches under our local rules).
He could have taken a reasonable break to deal with a clothing or equipment issue (but not a racket issue).
He could have taken a 5-minute bathroom break (and our local rules advise that you accompany him to make sure it is in fact a bathroom break and not a coaching or stalling break). Other local rules for timed matches prohibit bathroom breaks toward the end of the match.
If the opponent just up and left the court against your objection for some other reason, the thing to do would be to time him (using a cell phone, IMHO) and perhaps have a teammate do the same.
If it was instead a break for an authorized reason, then still time him. If you don't actually time him, then you wouldn't be able to win any grievance if you defaulted him.
If it is an excessively long break or an unauthorized one, then I would say at the time, "Hey, you're not allowed to take a break like this. If you go, I'm going to consider it your retirement."
-- Random Error Generator, Version 4.0
-- Master Moonballer