https://science.howstuffworks.com/explosion-land-water1.htm

Differential density. Your cavities (such as your lungs) and the gasses within get compressed when the shock wave hits and your tissues rupture when the pressure is transmitted through the water and through your body. Differential density is also the reason why things like shaped charges kill everyone inside a tank (using a shaped charge, a high explosive generates a shock wave which penetrates the armor of the tank via the Munroe effect, the energy of which is transferred to anything inside it as the pressure of the shock wave compresses and destroys your tissues) even if the tank armor remains intact (no or incomplete penetration).