Water Elemental is susceptible to Lullaby, Chant of Rage, Song of Love and Agony as well as it's attacks being avoidable worth armoring against.
Another "I win" vs a Water elemental is a guarding Plant creature or anything else with Hydro Immunity...including another Water Elemental.
Good ideas, Puddnhead, although I'm not sure I'd be willing to spend 5 mana (before any discounts) to gamble on Lullaby for two 50-50 chances at Stun conditions (and in the case of failure, two 50-50 chances at the Elemental not being able to attack via Daze). The math works out well on average, but it's scary leaving it up to chance in those "must not get hit/pushed this round" situations.
Along the lines of Agony, there's also Shrink, and anything that can deal Stagger tokens. Unfortunately Weak won't work, as it's a Poison condition.
Then there's Block, Glancing Blow, Redirect, Reverse Attack, Eye for an Eye, Divine Reversal, etc. They'll help to negate some damage, but generally won't affect the Push.
It's too bad [mwcard=DNE03] Stumble[/mwcard] and [mwcard=MWSTX2FFE05] Fumble[/mwcard] won't work on the Elemental (Unstoppable = Unmovable). If they worked, I'd seriously consider including them just as a counter for this guy - and a few other really big buddies. The same problem exists vs Titanodon; can't use Stumble to prevent him from Trampling, since he's also Unmovable.