Tools found show they were intelligent and made and used boats. People have a false assumption that neanderthals were stupid animals. On the contrary, they had a society and their own forms of communication. They are our history. The evidence speaks for itself for people who are open minded and scientific in their approach of knowledge.
Stone tools found on the island of Crete, which date back 130,000 years, prove that Neanderthals had the ability to travel by sea and use watercraft in transportation. According to an article in the New York Times in 2011, Crete has been an island for over five million years, meaning that the creatures that forged and utilized these tools had to come to the island via boat or some sort of watercraft. While the design of their crafts certainly would be crude, the vehicles that these Neanderthals used to travel to and from the island served the purpose of helping man discover new lands in some of the earliest forms of exploration. Researchers found over 2,000 stone artifacts which paints a picture of a widespread, organized movement to the island and suggests that the watercrafts that Neanderthals were constructing were at least stable and reliable enough to transport a significant amount of people to Crete, one of the largest islands just south of the Greek mainland.
Unless geologic evidence shows otherwise e.g. there was a land bridge or other event. I could believe Neanderthals made and used boats if the tools found on Crete are of the right time period. Also if the tools found are of the same style and materials found at Neanderthal sites, that would be convincing.
I think that the discovery of tools found on the island of Crete definitely is not enough evidence to prove that Neanderthals made and used boats. If circumstantial evidence was enough to "prove" something, a lot of incorrent things would supposedly be true... but that's simply not the case at all.