Bones need more than just calcium to be strong.  They also need vitamin D to better use calcium and help grow and maintain strong bones. Without vitamin D, your body only absorbs 10 to 15% of the calcium you intake. 

If you're using sunscreen, you may be inhibiting most of the sun's ultraviolet rays from stimulating your body's natural production of vitamin D. Don't stop going to the beach, just take a cooler of cold milk along. 
 
So for vitamin D, just remember: 

  +  =  Healthy bones and teeth