A clear, see-through curtain allows light to pass through, brightening up the bathing area. It also allows the person to see who's outside the shower area, a helpful feature if the person may be frightened when they hear noises but cannot identify the person outside the bathtub. A clear shower curtain also allows you to supervise from outside the tub area when the person resists your assistance.
For individuals who require privacy, however, an opaque curtain is the best choice.
Screw the curtain rod support securely into the wall for extra safety in the event that the person grabs the curtain for support.