First we would have to know how you are defining effective. Does your water activity burn as many calories as walking? Does it add more or less muscle density? Does it do more for overall cardio conditioning?
I rarely find an instance where one activity is “better” than another. Cross training your activities to get the benefit of a wide-ranging challenge to your body is what I would consider better. Do both, add some other stuff and have some fun learning new activities. You can teach your body to do just about anything so don’t get frustrated with a new activity, stick with it and get a well-conditioned body.