The cliff should not be your main focus, in my opinion. There is no chance that you will find any further physical evidence at that location. Sure, you should visit there a few hours so you have a good idea of what the location looks like when talking to others, but THAT should be the focus: communication with people who were involved with the situation.
Physical evidence is not going to be something you can get a hold of at this point. That’s gone. Instead, your visit to Taiwan should be a series of interviews. Find out the location of the police precinct that was in charge of the investigation. Make appointments to meet with anyone who had any involvement in the case.
Spend time with the ex-wife. Kindly but thoroughly interrogate her on all things leading up to Dave’s disappearance and following his disappearance. Talk to her family. Never come off as blaming them or they will immediately close all doors…and you will NOT be able to open them. Ask the wife about Dave’s lifestyle, who he worked with, who he hung out with. Trace those people down. Go to the diving community in Taidong. Ask around about Dave.