I went snorkeling alone off one of the Greek Islands without realizing that the tide was going out. When I looked up, I was miles from where I started. It seemed like it happened very quickly. A strong swimmer, I headed back to the beach - arriving about 5-6 hours after I left. Everyone assumed that I had drowned. Staying calm and focused is the way to survive that situation.
If this is what happened to Amelia, where she was in the water and unaware that the tide was going out, alcohol and night darkness would have made it very difficult for her to find her way back to shore. She would have removed her clothing to make it easier to swim, so even if she was found without clothing it can't be concluded that someone else removed her clothing. Furthermore, the ocean would have removed all DNA, and minor injuries to her body could have been caused by being in the water for a week.