The floating island / Haydon, Elizabeth

While doing a presale test of one of his family ships, Charles Magnus Ven Polypheme comes under attack by pirates, is rescued by mermaid-like creatures, and travels to Floating Island, where there are heavily sought after magical waters.

Attacked by pirates, lost at sea, and near death, Ven is rescued by a passing ship on it's way to the island of Serendair.