That timeline makes me nervous unless you are eating somewhere like The Wave, which is a little more quiet, but I always overplan for meal times, just because you never know if/how far they are running behind. For me, it would depend on where you want to have the meal. Any likes or dislikes? Casual or more formal? If you want 'Ohana, pick a time around 5:00 to 5:20ish to be safe. I know it sounds crazy, but they can run pretty far behind and the meal can last awhile. I'd rather have time to kill than be rushing to get to the boat.
I feel like 6:00 would probably be fine for anywhere else other than 'Ohana, though, the more I think about it. Most places can have you in and out in an hour to hour and a half.