Still Summer by Jacquelyn Mitchard

From the Publisher:
Back in high school, Tracy, Olivia, and Holly were known as The Godmothers, the girls everyone wanted to be and know. Unlike many friendships, their bond survived the years.

But 20 years later, their glamorous leader, Olivia, whose wealthy Italian husband has died, suggests they reunite on her return to the United States with a luxury sailboat crossing in the Caribbean. With Tracy's college-aged daughter and an attentive two man crew, they sail into paradise. But then, the smallest mistake triggers a series of devastating events. Suddenly in a desperate fight for survival, they battle the elements, dwindling food and water, the threat of modern-day piracy, and their own frailties.

Still Summer is at once a breathtaking adventure and a story about the bonds that hold friend to friend and mothers to daughters, and how facing our own mortality tests the truth of everything we think we know.