One way to compare them is to look at the average calories burnt. So as a broad example a person who weighs around 11 stone will burn about 422 calories per hour doing general aerobics, and around 281 per hour for water aerobics. Of course these figures will vary depending on weight, intensity of the activity and metabolism so are just a guide.
Since the resistance of water is higher than that of air, water aerobics is reported to be more efficient at training all muscle groups. Water aerobics has several advantages compared to regular aerobics. Water is easier on the body, it releases joints from their load, strengthens and stimulates the body and supports weight releasing the burden on joints and muscles. It is healthy exercise and particularly good at working the muscles which - because of the water - will not hurt too much afterwards. It is also meant to be good for women who are pregnant because of the way the water supports the body.
For healthier weight loss whilst strengthening the muscles you may need to combine it with some cardio exercises � or normal aerobics if you like. As said above, any exercise that burns slightly more energy than you consume is good until you reach your preferred weight, just be sensible with it and the eating and try to mix and match activities to suit.