The SmartSwim UV Intensity Bikini ($99) has little purple beads right between the tits (and other, more boring places) to let you know how intense the UV rays are. Light purple good, dark purple, cancer. I suppose if you don't want to shell out 99 bones you could buy any bikini and add your own beads, but that would be cheating. And cheating, my friends, wins games.
SmartSwim UV Intensity Bikini warns against excessive exposure [dvice]