Sorry this answer couldn't get to you before the movie started last night, but no, you don't have to get in the pool to watch, as they have several screens angled for swimmers as well as non-swimmers. I would recommend getting there when the waterpark closes (at 8, which is an hour and a half before the movie starts). It wasn't so necessary to get there that early last night due to there not being a lot of guests, but there normally is a pretty good amount, so seats can fill up quickly.
Did you end up going last night?