Just wanted to add my experience of my aquanatal class. Being a keen swimmer I tried my first aquanatal and felt I could have done with a bit more of a work out(I actually did extra laps when the class was over). However I love being in water and any exercise is good. I noticed when we finished, on my way out the next class was the over 50's and they were doing the exact same exercises lol! So for those who don't have an antenatal swim class or similar, it may be worth considering doing a normal aqua aerobics class and if the instructor will allow and just taking it at your own stride. As its in water its unlikely we do any movements that will be risky for baby anyway.
p.s on the plus note having all the bumps around in water gives you a chance to socialise with other expectant mummies