Fathomless Pursuit by Eric Kercher

Ravaged by pirates, starvation looms for the crew of the submarine Alopias. Now responsibility falls on Christos Maris, second in command, to assume the mantle of leadership.
But roving bands of pirates are the least of his worries. Their submarine, built by the mysterious Shipmasons long ago, is dying. If they don’t save her, their lives will end too. Whispers of a legendary submarine, built just before the Flooding, give them hope they can save her.
But the deep doesn’t give up its secrets lightly, and Christos must use everything he’s learned to survive. His friends, family, and a shipwrecked girl, are putting their lives in his hands.
Dangers lurk in the deep that can swallow them whole, and there’s no certainty this treasure even exists. Will his gamble to protect their way of life pay off?