In the part after Baby says she wants to "drop him on it" when practicing the lift in the water... she fixes the strap on her shirt... Johnny is supposed to do this in the script, but instead she does it. If you look, a few seconds later, Patrick Swayze tries to do it, but realizes that she has already done it... oops, too late.
No, I think this is all part of the script; they both go to do it at the same time, but Baby gets there first... to increase the forthcoming sexual tension perhaps? Anyway, no slip up.
Actually the scene where she says "she's going to drop him on it," is not when they are practicing in the water. It is a few scenes before when they are going over the dance routine in the dance studio (or somewhere at the place they are staying). The scene goes something like this: Baby and Johnny are going over the routine at the end they are supposed to kneel down on the floor and bend backward w/ their arms around each other. Baby almost makes him fall so he says," what are you trying to do kill me?" then she goes into a long smart-alec response that includes that line.
Natalie was right. With the music in the background, the sneaky glances back and forth...all of that in addition to the whole helping put Baby's shirt strap up was to add to the sexual tension.