no matter how durable your opponent is, they still need to breathe (with a couple exceptions like sasori.) a decent suiton user could take down a much more powerful opponent if they manage to drown them.
Water should be the element with the most potential, but Kishi doesn't see this:erm:. With the force that Shinobi use Suiton, it should at the very least cause some internal damage to the body.
You really think any half decent shinobi would fall for drowning just like that? ._.
It would take every Suiton user Kisame like prowess to attemp something like that.