One hair is not enough evidence she died in the boat. May not even be enough evidence she was there. If you steal clothes from someone for instance likely there can be their hairs in your home.
That is why the overall web of circumstantial evidence is what proves murder - by inference.
A water taxi driver confirmed
1. He delivered the teens to their boat late that night, but it was full
2. A man, on his own, offered the teens to sleep on his boat
3, He delivered the 3 of them to an unknown boat (but was confused about whether the boat was a sloop or a ketch)
Then
4. The teens were never seen again
5. All other boats were eliminated as to boats they might have been delivered to
6. The accused set sail early on NYD
7. Victims hair found on his boat
8. An anchor was missing
9. The accused was alone on his boat
10. He was seen out in deep water
11. He then changed the look of his boat (repainting)
etc etc
A photo of the victim, not seen for 13 years is like the hair on the boat. Brutally incriminating IMO.
This is why I think they don't have it, but rather, a witness account of something like that