You are wrong. They got off the ship in Curacao after being assured every inch of the ship had been searched. They thought they she had to be on the island so they got and stayed and searched for her. Eventually, they went home and came right back to search for her.If my daughter had gone missing on a cruise boat and I believed that she had been kidnapped and possibly trafficked, there is no way known I would immediately go back home.
This is what the Bradley's did...

'Amy Bradley Is Missing': Where Is Amy Bradley’s Family Now?
More than 27 years after Amy Bradley vanished from a cruise ship, her family still holds out hope — and remains at the center of a case with no clear ending.

Brad Bradley and his father flew back to Curaçao shortly after the end of their vacation to search for her themselves, they describe in the documentary series.