I've not heard that any sunscreens are particularly better for the reef than any others (most organic sunscreens are simply due to manufacturing and ingredient reasons, not because the resulting compound is necessarily better for the environment). The best tip is to apply the sunscreen at least 30 minutes prior to going in the water. This allows the sunscreen to penetrate the skin and avoid leaving behind the "oil slick" that can happen when you apply and go in the water immediately. Does that help? And yes, using a rash guard or a dive skin would definitely be even better in terms of protecting your torso - but you'd still have the issues with your face, neck, arms (hands) and legs (feet).
Masaki