Umi no Misaki (Messanger of the Sea) (Patreon)

A comic series from some time ago, presumably set in Miyako Island (that island name is never mentioned, but background scenery is fairly revealing). It features three beautiful native shrine maidens, the main heroine Shizuku (pictured center) being the sea shrine maiden. I initially thought the story to be too outlandish, but after some further research about islands of Miyako I am beginning to see some authenticity.

And for those who are curious, there is a fair amount of freediving and yes the girls do engage in a breath holding match. :-)