Castaway Heat by Kelex

Shiloh Anderson fell in love with Ronin Drake when he was only fourteen years old. Unfortunately, the twenty-year-old alpha wasn’t too pleased to have a young omega following him around like a lovesick puppy. After Shiloh’s young, tender heart is broken—coupled with memories of his parents’ very nasty divorce—he realizes that relationships are simply more trouble than they’re worth.
Nearly a decade passes without Shiloh seeing Ronin again. When they end up on a South Pacific cruise together, Shiloh’s no longer the one doing the chasing. While his inner fourteen-year-old is cheering, Shiloh’s not the easy target he’d once been.
When the pair end up shipwrecked together on a deserted island, avoiding one another is no longer possible. They have to work together to survive while awaiting rescue. As the days pass, they edge closer and closer to danger.

All they can do is pray help comes before Shiloh’s heat does.