Frankly every valley is basically made by water eroding the softer ground away. So even those places where no river or stream there should be some periodic water drainage.
All the sandy beaches in a place like Kyushu's SE shores are the work of rivers? They are basically alluvium (alluvia??).
Plus pre-industrial Japs without tap water had to drink something. I dunno how wells fare that close to the sea.
Bah, it doesn't matter for my amateur typology of Jap fishing villages.