Cause for round holes in stones
Your image looks very similar to rocks we get here on the West Coast of Vancouver Island.  We have some rock boring bivalves that will effectively drill into the rock which breaks off and erodes into a shape similar to what you are showing.  https://en.wikipedia.org/wiki/Pholadidae or https://the-hazel-tree.com/2013/08/21/piddocks-anything-but-boring/

There are also echinoderms that will bore into rocks, Strongylocentrotus purpuratus, https://en.wikipedia.org/wiki/Strongylocentrotus_purpuratus 
https://bmscblog.wordpress.com/2013/01/09/holey-rocks/  but the holes from these creatures tend to be larger than the image you present.

I a confident that the holes are created by some form a boring bivalve.