Tell your instructers that you are preganent, and keep your kicks low. Listen to your body, and dont over do it, or over heat. Drink plenty of fluid.
I do water aerobics, and that has been fine, i just tone down a few bits to suit, I dont rebound, do high side kicks etc, or to much that uses my stomach muscles. .
With anything, specially with kicking etc, keep an eye on other peoples. I near got kicked in the belly doing water aerobics.