The Prophet said, Martyrs are those who die because of drowning, ...

Hadith 1:688

Narrated Abu Huraira:

The Prophet said, "Martyrs are those who die because of drowning, plague, an abdominal disease, or of being buried alive by a falling building." And then he added, "If the people knew the Reward for the Zuhr prayer in its early time, they would race for it. If they knew the reward for the 'Isha' and the Fajr prayers in congregation, they would join them even if they had to crawl. If they knew the reward for the first row, they would draw lots for it."