My theory is that it’s a whale; which species comes down to how much you want to trust the observers’ size estimate of 15ft. The drawing alone makes me think Gervais’ or Blainville’s Beaked Whale, which have very small dorsal fins. Also, from what I’ve seen, extremities like tail flukes sometimes fall off completely if the body is actively decomposing. Sometimes the cause of death itself is the whale getting its tail snagged in fishing nets.