The instructor had us walk around the perimeter of the pool once, then go around again - backwards. That's harder than it sounds. We used Styrofoam weights and "noodles" against the resistance of the water, pushing and pulling until my biceps quivered with exhaustion. We scissored our legs back and forth. We lunged and lifted and loped. By the time I pulled myself up the pool's steps an hour later, my legs felt like wet bags of sand. I staggered to the whirlpool hot tub and collapsed.
The next morning every muscle I possessed reminded me of the night before. Water aerobics may be low impact, but I can testify that it isn't low energy. And it's definitely not as easy as it looks, especially if you're 64 years old and NOT pregnant.