Early Eurasians had boats 50 ka which is the best explanation. Africans still tend to be afraid of water, and Neanderthals would have made horrible swimmers so they were most likely hydrophobic.
Early Eurasians likely had nets, and a diet rich in fish is known to increase intelligence. Humans could cross large rivers while Neanderthals probably could not. I'm not sure what the caloric advantage of fishing in a boat with nets is, but I bet it is massive if you do the math, and Neanderthals would have had difficulty duplicating the behavior if they were hydrophobic.