Hoomins be smart motherfuckers. what's a little ocean for the smartest monkey in existence? It's not like paleolithic folks were dumb, and we probably have a very bad idea of the level of tech paleolithic and neolithic peoples had on account of, well, we think of them as "stone age" when stone was probably a marginal material for them: most of their stuff would have more likely than not be made of rope, wood and leather, which are more workable and plentiful materials: it's just they rot, so we don't see them.

the Rapa Nui people had these highly complicated seafaring vessels made of straw and wood and i don't know what else, and it's been established that those things could travel long long distances... and i mean, the proof is that easter island is populated since a long time ago: no reason to think neolithics were any worse at seafaring or any less into fishing or oceanic adventures.