I do not think his plans were to intentionally leave with someone. I believe, if that was the case, he would have TOLD his cousin that he was leaving. I am pretty sure that most people would agree with me here- if you are out with a friend and you decide to leave a crowded bar, you would TELL the person that you are with where you are going. I think it would be out of character for MOST people not to do so. Of course, intoxication is a factor- I do have one friend that leaves the bar without saying anything .
As I have speculated ALL along- I believe that Joey was conversing with SOMEONE that night, whether that be through social media or texts- I think that this person was in the area and let Joey know that he was at the bar or outside the bar. I think Joey left to meet this person, not thinking that he would not return back to his cousin within a few minutes. It wasn't intentional for him to leave without telling anyone. He didn't accidentally drown- he didn't drown at all. He was killed and disposed of in the Scioto.
I keep thinking that this person was in a car, parked in the alley right next to the Union- I believe this person told Joey to hop in the car and "help" him park (seeing as though it was densely populated with tourists that night- not to mention, you can't easily park in the Short North on any given weekend)- this person is probably not familiar with the Short North- this could explain the "I'm driving" text- he was driving, but he was in a car with someone.
I don't believe he left that bar on his own accord and started walking towards his car- whatever happened, happened either INSIDE that bar or right outside the bar.