I have been trying to throw these also with about the same result I have been trying to find a good video were it was thrown really slow but have had no luck. I am going to have some one video tape me this week end and see if I can see what I am doing wrong.
I cant land these on the water yet but when I started doing them on the trampoline I would have that same problem if I brought my lead hand (rope hand) down towards my side during my rotation. I started video taping myself and immediately noticed my problem. After that I would concentrate on keeping my rope hand above my head during my rotation and can land them every time on the trampoline now. I think you should get yourself on video. If you know what theyre supposed to look like you should be able to see what you are doing wrong.