Just some of my random thoughts and questions on things that have been mentioned so far:
1- In the many tourist photos of this caye, I don't see any lights and since any electricity would have to be generated - I would suspect no installed lights (plus the mention of using a flash light to find the toilet). Jan 18th was a quarter to crescent moon, so not a lot of extra light.
2- Many tourists mention a small food stand on the island with beer. I would be more suspicious if there were mixed alcoholic drinks or if anyone else on the tour mentioned either bad alcohol or extra strong drinks. While possible, it doesn't seem super-likely that she was heavily intoxicated. (all MOO here).
3- I would suspect that the toilets have buckets of water "ready to go" for the evening, but there still could have been a need to fill one up which would explain a trip to the water. However, wouldn't a bucket be missing or found floating if this scenario occurred?
I have no idea the challenges of finding a body in the ocean - from a google map view, there appear to be lots of sandbars and barrier islands/cayes around - I would think the locals would know if she would be pushed towards any of those based on wind and currents.
I really hope she is found but it doesn't seem likely if they are calling off the search. I wonder if with all the tourist boats around, some clues may still be found.
While I am glad the FBI is investigating, seems like a job where the US Coast Guard could have been of more assistance