Pretty much what has been said.... there are no truly viable templates for personal watercraft. Or 4 wheelers, most motorcycles, snowmobiles, boats, etc. We have done lots of PWC's and boats since we have a huge lake here.
Take a lot of photos and measurments, then either draw it on paper by hand or for the highest accuracy cover what is going to be decaled with paper app tape, pencil the outline. Carefully remove that and stick it to a flat wall, stand way back and photograph that. Then correct the photo for lens distortion, open in Illy and trace the outline adjusting measurements from the actual pencil tracing.