The Tree of Blood! Part-1.

Oaklum, City outskirts, seashore.

Next Day, Morning.

"So, how are we going to go underwater?" (Riya)

"Hmm, I and Rio likely don't need to breathe oxygen, so we can swim without worrying about drowning…" (Link)

A-rankers are partial superhumans, they can use mana in place of oxygen to supply energy to their cells, and thus they don't really need to breathe at all, making them immune to drowning due to lack of oxygen.

(S-rankers and above-level individuals don't need to breathe, eat, or sleep at all. As they have already broken through their racial limits they could easily live on mana alone…)

This means that Zach, Riko, Rio, and Link all four of them could directly swim to that underwater cave without worrying about the lack of oxygen and complete the mission all by themselves.

Moreover, they can use mana to protect themselves from the underwater pressure