score:8
There is not really a reason to doubt that Neandertals were capable of short travels over water, though clear evidence in form of a datable stratigraphy and correlation with contemporary sea water table is still lacking. Insofar, saying that they did sail the open waters is still a bit premature. No remains of a boat, raft or floating device from Neandertal times have been found until today.
https://advances.sciencemag.org/content/5/10/eaax0997,
and
https://www.sciencemag.org/news/2018/04/neandertals-stone-age-people-may-have-voyaged-mediterranean.
It fits into the ever improving picture we get from our relatives in the human lineage. Work was also published showing that Neandertals were capable of symbolic behaviour (burials, personal adornment, painted objects) since the Eemian. (paywalled: https://www.nature.com/articles/s41559-018-0598-z)
That they made composite tools at the time and maybe earlier is known since long, for example from find sites KΓΆnigsaue (reproted in the early 1970s), Campitello, Inden-Altdorf, and experimental archaeology:
https://www.nature.com/articles/s41598-017-08106-7
and just recently arrived, evidence of cord fabrication from plant fibre:
https://www.nature.com/articles/s41598-020-61839-w
Several paleolithic find sites exist with wood preservation (for example Aranbaltza, Clacton-on-Sea, Poggetti Vecchi, Abric Romani), even one from Homo erectus times (SchΓΆningen). Technology and knowledge would have been there in the times of oxygen isotope stage 4 and the first half of 3 (80,000-40,000BP), but in a given case of a find site on an island one would have to correlate sea level stand and bathymetry, which is not a big problem these days, provided an exact age dating is possible.