Geronimo Stilton # 62:Mouse Overboard

My family and I, Geronimo Stilton, were invited to Lisbon, Portugal, to retrace the journey of the great explorer Vasco da Gama. He was the first European to reach India by sea -- and was apparently an ancestor of the Stiltons! But once we got on the boat, I learned that someone was trying to sabotage the voyage. Could I figure out who?