How most games do this is similar to how they tell if you fall out of the map versus die by an enemy. Create a new Damage Type and call it Water or Drowning Damage Type. Set the water to use that damage type. When the player takes damage, run a branch check to see if the damage type == water/drowning. If yes, then they died by water, if no, they did not. You can even do a Switch on the damage type on the AnyDamage function and set a different reaction for each type of damage. Let me know if you need more information.
Learn more about Damage Types, what they do, and how to use them.