There most definitely is water on both sides....
The Kaiju are aquatic, they swim up to and through the breach.
The Escape pods clearly rise due to the flotations devices attached to them when ejected from the Jaeger, which would indicate the presence of water.
The Jaeger clearly drifted down through the water when it was sinking into the Anteverse, as opposed to its fall from 50,000ft.
& the icing on the cake.... when the reactor is going to explode, in the last scenes before the escape, you can clearly see water rushing into the mech from damage to the exterior. Will one of the above posters, claiming that there isn't water on the other side of the breach, please explain why there would be water filling the mech and spraying around inside the Jaeger if it was not submerged? (Don't tell me its left over from the ocean, it is clearly under pressure when sinking to the bottom of the Anteverse.)
That's a wrap!
P.s. I just watched the movie again, when Mako's escape pod is released, a bunch of bubbles can clearly be seen escaping the Jaeger with the pod.