Ambassador 5: Blue Diamond Sky. Ambassador: Science Fiction Thriller Series, #5

As Cory takes a well-earned rest and finally submits to proper weapons training, he and a couple of people from his household go on a hunting trip in the marshland between Barresh and the turquoise sea. A bad storm has come through recently and on a deserted beach, Cory finds something Earthly that doesn't belong there: a message in a bottle, a piece of paper with HELP scratched on it with a sharp object.
In Isla. Cory has a list of all humans in Barresh: it's very short and no one is missing. A few days later, he receives a curious message through official channels, from a woman on Earth whose rich businessman husband went on a trip of a lifetime "in a place where you can surf with plesiosaurs in turquoise waves". Cory knows the guy advertising the trip. He's a shady character. He also knows where the "plesiosaurs" are.
They're not particularly friendly. Not to mention that the area is on the land of a viciously territorial Pengali tribe. As it turns out, those are the least of his worries.