My High School Physical Education instructor repeatedly mentioned that a workout in a pool is more beneficial, for the additional resistance but the little feeling of weightlessness. This Underwater Treadmill may not be fully submerged, but provides a great solution for rehabilitation and exercise.
Since the body is submerged from the waist down, it has additional resistance when walking. For patients going through rehabilitation, it will still provide the workout while keeping them somewhat afloat. You could always argue the case claiming that patients should strain more when recovering, but I think that is on a case to case basis.
I am not sure if exercise machines are becoming easier or just more fun. A fun example could always be the Anti-Gravity Treadmill.