Puerto Rico [1] is perfectly safe for women traveling alone or in groups. But precautions should be taken. Dressing provocatively can attract catcalls and other forms of unwanted attention, and bathing suits should never be worn anywhere but at the beach or pool. Women traveling solo should avoid remote wilderness beaches and late-night bars that cater primarily to locals. Safety in numbers is a good rule to follow when going out at night.
Links:
[1] http://www.moon.com/destinations/puerto-rico