Book 2 in the Omega Sub series. The USS Liberator heads to the south pacific to look for an island to establish a base. They have brief encounter with the unknow submarine they've called the Nemesis. There's some brief excursions to some islands. With differing results. And then they discover a Russian sub has been quietly tracking them and then it attacks.
The main plot line is never accomplished. There's a lot of side plots and a tiny amount of character development. But it devolves a bit into a Hunt for Red October situation.