The Thalassa is the beautiful Tall Ship with which School at Sea sails every year.

A Tall Ship is a large sailing ship. The Thalassa has three masts and is one of the most beautiful and fastest sailing ships of the Dutch fleet. On board are a captain, a helmsman, crew members, teachers, a ship doctor and a cook, who will accompany us on our way to independence.  

The name 'Thalassa' comes from the Greek goddess Thalassa, the goddess of the sea. The Greek word  Θάλασσα , therefore means 'sea'.

source - schoolatsea.com