If I'm having clear, watery stools, does that mean I'm done with the prep for my colonoscopy?

Usually. This the endpoint we are striving for. If however in the morning you pass anything more than clear liquid, let the nurse know immediately on check in so they may give you an enema or two to permit completion of the colonoscopy without you having it cancelled and rescheduled and needing to go thru the prep once again.
Yes. Clear, liquid "stool" is the ideal end result of the prep for colonoscopy.