I did the Leopold quest once, only I didn't rescue Ash. When I finished, my log said I had rescued Johansen, ans when I got to the boat, he was standing on the dock.
I went back and did the Ash rescue, and then redid the shootout at the OK Corral with the nutty priest, but now Johansen will not follow me to the dock. I've tried several times, and he just stands there. Is there a way to trigger him following you, or does it matter whether you save him? I'm getting sick of this sequence.
After you board the boat, does it register as a failed mission or does it say you've rescued him? Regardless, it doesn't matter in terms of advancing the plot. You've already got the information you need from him by that point.