Minor Water Elemental summon (from Lord of Sea spell) has no special abilities in the game.
But their XML entry, in CoreSummonedUnits.xml, lists DrowningAttack.
So why it doesn't appear in the game?
Because that is wrong ability name. It is supposed to be DrowningStrike.
If that is changed, ability will properly appear for them. Exempt that, essence of the Elemental should be increased to 3, or otherwise they would have no mana to use it (DrowningStrike costs 3 mana).