There are specific rules covering drowning in the Dungeon Master's Guide at least for players (but it isn't a far stretch to apply them to monsters), but your DM has called it on the spot that the creature has just perished in the water.
If it were a player/NPC/monster that was in some way critical to the story/encounter, then it may have been preferable to allow it time to drown and thus time to save itself or to be saved by someone else, but in any other case it is perfectly valid both for the flow of the narrative and the entertainment-factor to just rule that your smart or lucky playing has ended in a result that benefits the party without looking up rules and thus slowing things down.
The actual rule (for player characters) is that during strenuous situations they must succeed a DC20 Endurance check every turn on which they take damage to prevent the loss of a healing surge. Once the character has no healing surges remaining they take their level in damage instead. I (personally) would also say that monsters follow these rules, although they may not have healing surges so will take damage from drowning sooner.