I know that when we were there in July and MK closed at 1 am, the boat captain had announced that the boat would be running until 3 am. So, the norm is probably 1-2 hours after parks closing.
sorry all after I posted this I found the info on the WL faq thread and it says about a hour and half after park closing and the boat from cr to wl is 6:45 to 11:30, so even if the park closes early we could go that route. Thanks everyone!