Harbinger. Triton Core, #2

Trusting the lethal mercenary is complicated. Falling for him is easy. Marine biologist, Dr. Kathrine Sorland barely survived her first marriage. Now, her dog is the only male she has time for. When protester activity threatens her research, monitoring a quarantine zone in the Norwegian Sea, Kathrine's scientific crew comes under the protection of a covert black-ops team--the Ocean Wolves. Working with their commander is a challenge, not least because he reignites feelings in her heart she believed long dead.
Formerly in the Russian Navy, Nik Borostovlo is in command of the Wolves for the first time. Married to his job, responsibility weighs heavy on his shoulders. There's no room in his life for a woman, but no matter how hard he tries, he can't deny his attraction to the strong-willed scientist. With every passing hour, trapped in close proximity on the dive ship, her presence makes him remember what it is to feel.
As events escalate deep under the polar ice, it becomes clear the quarantine may have been breached and a deadly bacteria unleashed into the world's oceans. Nik may be the only person Kathrine can trust, and time is running out to keep it from the wrong hands. Thwarted by a traitor in their midst and impossible choices, Nik will face his toughest challenge yet as he fights to protect the woman who's captured his heart.
Harbinger is the second book in the series, 77K long. Each book is a standalone romance, which follows an overarching story. They can be read in isolation but for maximum enjoyment are best read in sequence.