In the movie, the ocean did not exist before Murdoch conquered the Strangers. The Strangers designed the city's air pocket to be contained by a force field that ended within the boundary of the Shell Beach outer wall, the one to which you refer. This wall, when breached, was not selective; anything passing beyond it was lost in the vacuum of space. The fact that the wall could be breached at all suggests that the Strangers were being lazy and not incorporating extra reinforcements into the solid wall portion of the barrier, but saving their efforts for keeping the air contained by the 'clear' portion of the force field above the city.
However, when Murdoch is brought fully into his 'tuning' ability by Dr. Schreber's memory injection, he acquires what is essentially godlike power, and is capable of changing the city's physical constants. After winning against the Strangers, he does so by 1. widening the force field and air pocket to extend beyond the city walls, making it large enough to 2. include a newly created ocean outside the previous wall boundary. He then tilts the habitat toward the sun, allowing sunlight into the Dark City. (reference)
The story was not inconsistent; the boundary/force field itself was shifted to contain a larger area once Murdoch was in control of the Dark City. The new shape of the force field appears to be an oblate spheroid extending well past the original wall, but not substantially taller or deeper than it had been before. Hopefully Murdoch, as a benign demigod, incorporates measures to ensure that people don't actually sail or swim out into vacuum past the ocean, perhaps simply by making the force field impermeable to all matter. Since the force field no longer appears to be contiguous with any solid barriers, it is likely that this is the case.
