I do Aqua Zumba now on a Monday evening and it's a good laugh.
The instructor is good but she goes at 100mph and I think she forgets that you cannot move that fast in water! We all end up doing our own thing about 5 paces behind.
Not sure I'd have the confidence to go to a class though. I've got it for the Wii and keep saying I'll try it out first before I embarrass myself...
It is absolutely hilarious, you WILL enjoy yourself but you will also wake up tomorrow thinking someone has cut off your ankles.