Glaucos

Glaucos (グロコス) is a completed manga series by Akio Tanaka that was serialized in Morning.

Plot
This is a story of a boy who challenges free diving. "Free diving (skin diving)" may remind you of the film "Le Grand Bleu." It is an extreme sport, and each meter added to its record brings us closer to the sea bed and further from the surface where one can breathe again. In this sense, free diving has a direct connection with death, with which our fight continues, refashioned over time, bit by bit. So, why would one free dive, and risk his life? The answer lies within the mysterious bond between the sea and human kind. And in the fast evolving world of competitive diving, new data emerges that casts light on this bond.