When a boat or other vehicle comes by, both of them scream at it. The size of the vehicle influences the volume of the scream. If the vehicle is small, SpongeBob and Patrick scream quietly. However, if the vehicle is large, SpongeBob and Patrick scream loudly.
Role in episode
After SpongeBob and Patrick exit their respective houses, they start playing Boat Screaming. They start screaming at every vehicle that passes them, eventually using megaphones while playing, which causes Squidward to angrily yell at them to stop.