I agree with you that the neandertals were fully human, but sapiens and
neandertals differ and the neandertal skeleton clearly retained or showed
more diving adaptations (denser bones, ear exostoses, heavier body...) than
our skeleton does. There are several independent indications that (some
groups of) neandertals might have relied not only on terrestrial resources,
but also, and more than most modern humans do, on water resources.
Mary Stiner, in a study of Italian cave sites, showed that in her study most
of the terrestrial small species were prey elements in the diets of a
variety of small carnivores and raptors, but most of the marine molluscs and
turtles there were collected and eaten by neandertals (M. C. Stiner 1995
"Honor among Thieves" Princeton UP, NJ).
No doubt, many neandertal males frequently dived (note that I'm not even
claiming any more than all neandertal populations dived). That is shown by
the presence of extensive and bilateral auditory exostoses in male
neandertal skulls such as those of Shanidar I in Iraq and La
Chapelle-aux-Saints in France (G. E. Kennedy 1986, AJPA 71:401). Auditory
exostoses, bony swellings of the ear canal, a condition well-known to
otolaryngologists, occur exclusively as a direct result of long-term
exposure to relatively cold water (P. H. Rhys Evans 1992,
J.Lar.Otol.106:214), and are seen in all human populations that exploit
either marine or freshwater resources (usually shellfish) through diving in
water colder than about 18°C (Kennedy 1986).

Be realistic instead of letting the neandertals do what you want.
Or do you think it was not me who put their ear exostoses in their skulls?