I was very tempted to use Extract Water Elemental's image here. We need something more interesting than waves with faces!
Small Water Elemental
Unsurprisingly, water elementals are swift swimmers. They have a pretty neat natural armor bonus, bonuses to melee stats, the ability to extinguish fires both magical and mundane, and the horribad Water Mastery. -4 to attack and damage rolls if your foe is touching the ground is very annoying.
Water elementals can also turn into vortices, which is much like an air elemental's whirlwind (to the point that it uses Whirlwind's terminology in places), but ultimately much less useful as it only works underwater.
I suppose water elementals may be worth playing in a handful of games, though. +0 LA, but only barely.
Medium Water Elemental
Vortex is inferior to Whirlwind, melee ability is inferior to an earth elemental's... I'm not seeing any reasons to play those, especially not with extra HD. -0 LA.
Large Water Elemental
So yeah, everyone knows elementals are doomed to -0 LA from large size on, so I'm very surprised if anyone is still reading this.
Huge Water Elemental
On the upside, I looked ahead and there doesn't seem to be another string of similar, high-HD, low-CR monsters for quite some time.
-0 LA, by the way.
Greater Water Elemental
Doesn't seem that great to me. -0 LA.
Elder Water Elemental
I am seriously wondering whether putting '-0 LA' here is worth the effort. I mean: it's not like anyone doesn't know it already.
And with that, we finish the last elemental! Next are elves: that ought to be easy.