Got the shark to move correctly by deleting the extra costumes that were made for it (there were costumes to depict the shark facing left in addition to the default ones facing right) and just programmed the shark to face left when the left arrow is pressed, rather than changing to a different costume. Also needed to change the rotation style to left-right so that the shark wouldn't flip upside-down when turning left or right, but returning it to all-around when moving up or down.
