Water Elemental at max tells us in the tooltip that it has a 25% chance to deal 30% damage and drench enemies for 5 seconds.
Testing on Harpies show mine hitting for 12,014's and the proc doing only 2,241 damage.
Testing on Ram's show 59,283 and the proc doing 2,212 damage.
Harpies and Ram's were rank 1. Medallion used: Def Power: 5643, Def Speed: 2351, Def Health 2351. Shards: Destruction, Vampiric Empowerment, Water Elemental. (All maxed: 34%/60%/30% respectively)
Looks like it is currently doing 30% of the base Defense Power rather than 30% of the actual tower damage.