A grenade blast underwater is more deadly than on land.
If you had to choose between facing a grenade on land or in water which would you pick? If you picked water, then you’re dead. While facing off against a grenade on land isn’t great either, the explosion underwater sends a shockwave with enough pressure to crush your lungs or lead to brain hemorrhaging. You’ve got a little better chance ducking for cover on land. Let’s just hope we never are faced with this scenario in the first place.