Exercise in the water with your baby at your side in a challenging, five-week class at the Leventhal-Sidman Jewish Community Center (333 Nahanton Street) in Newton.  Mothers will focus on aerobic conditioning while babies (15 pounds to 24 months) float in flotation seats. Babies should be able to hold head up on their own. Pregnant moms welcome to attend. The class takes place on Sundays (4:30-5:15pm) from March 16-April 13. Cost is $75. Register at www.ohbabyfitness.com.  Free trial for the class will take place on Sunday, March 9 at 4:30pm.