The Dragon in the Sea

From back cover Del Rey paperback September 1978:

FOUR MEN
Were on board the atomic subtug Fenian Ram S1881. They were on a mission to steal vitally needed oil from underwater deposits in enemy territory -- a mission from which none of the last twenty tugs had returned...

FOUR MEN
Fighting a war a mile and half under the ocean, isolated by the unrelenting pressure of the water and by their own hellish fears...

FOUR MEN
Who knew everything they had to know about one another -- except which one of them was the saboteur who could destroy them all!