Given that Van Der Sloot is now under suspicion of human trafficking in Thailand where he now lives, it's not implausible:
One thing about the entire case that always struck me was the idea of Natalie Holloway partying and drinking with a strange guy to the point where she was intoxicated.
Strange things happen on vacation, especially in a place like Aruba, but she was an honors student and her family and friends have all stated that kind of behavior was very much unlike her.
What I suspect is that she was slipped a rohypnol or GHB at the bar and then taken to the beach where she either died from an overdose and was dumped in the sea or was transferred to a boat as Van Der Sloot describes...'drunk' and unresisting.
UPDATE: In yet another twist on this sad tale, Van Der Sloot now claims he was just pulling Greta Van susteren's leg and made the whole thing up. This would be the third or fourth time he's changed his story..