tracked more than ch were more than 50% likely to stay active than children who didn't participate or pecialized in only one spor. >>> and the latest trend in water aerobics works your core and gives you a dose of fantasy at the same time. danielle nottingham shows us a unique under-the-sea core workout. >> reporter: at the hotel dell coronado in san diego, a mythical creature is coming to life. >> my favorite part is i actually get to be a mermaid. >> reporter: holly isn't shy about putting on these colorful fins and diving under the water for this fitness workout. >> it was more fun than cold have anticipated. >> reporter: splashing around in fish tails may be fun, but isolating your legs and arms is also intense training. >> part of that entails getting your heart rate up, get their core activate and working to get their biceps or triceps, upper body. mermaids. even mermen like taylor raines are giving it a try. >> what kind of workout did you get? >> i would say it's a good swim workout and a fair amount of abs workout. like my abs are hurting, sitting up straight. >> reporter: holly says th