Sayyidah Umm Haram (may Allah be pleased with her) says: The Messenger of Allah (peace and blessings be upon him) said: "If one travelling by sea suffers vomiting (seasickness), he has the reward of one martyr, and the one who drowns has the reward of two martyrs."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8668