You don't need to accurately record increase in numbers to understand that movie nights are always more popular than standard opening hour days.
That's the other thing, putting the movies on entices not only increased attendance, but increased Annual Pass sales, I doubt anyone would pay for a ticket just to go in the afternoon for the movie, and passholders not spending anything in-park will always be a problem (A management problem, that is) but there will always be people ready and willing to buy the park food as you can plainly see from the constant long lines during most peak times of the day.
As you said, you can't accurately track the numbers, and it's pretty obvious that's the only reason Dive-In is going the distance, there's no concrete way to tell just how much impact the movies are having on attendance, only the fact that they obviously do affect the numbers positively (We just don't know how much) and for that reason they'd likely prefer to have special events like Fright Nights instead.
I stay grounded in my opinion that there is no way the Dive-in Movies did not affect numbers and revenue positively, but I understand why it's going.
Although I think they really should have just closed the waterslides once the movie starts so there'd be little extra running costs.