I agree with STS. I think you should wear a speedo type suit whose sides are not too narrow - I wouldn't go below 2 or 2.5". If you were to wear even moderately short swim shorts, you'll probably feel the difference in mobility between that and a swim brief since a brief is what you're used to. I also swim while a water aerobics class is going on and regret that I can't take one because I wear hearing aids and glasses and wouldn't be able to hear the instructor or see what others are doing.
Enjoy the class.
D67