Because water and electricity don't mix, and salt water is an even better conductor than fresh water. When salt water covers a lithium battery, it can cause a short circuit that causes the battery cells to overheat in a sequence of rapid reactions, a process called thermal runaway that can cause the entire battery to burn.
The tragic incident highlights the dangers of human smuggling attempts, particularly in treacherous waters, and emphasizes the ongoing challenges faced by families seeking a better life.