A recent experiment on the ISS showed that plant growth in space, with help from directional light, is entirely possible. The findings show that the plants grown in space were smaller than the equally aged ground control counterpart, but this can no doubt be accounted for when considering something like this.
This source (an ask-a-scientist program by the Argonne National Lab) gives us some great information on the amount of oxygen an average adult human breathes per hour, as well as the amount of oxygen an individual leaf produces. With all this taken into account, they estimated that 300-400 plants were necessary to sustain one human.
This estimate assumes 30 leaves per plant, which is reasonable.
If we generously assume that each plant takes up a cubic foot of space, then the maximum amount of space needed for a two-man crew would be 800 cubic feet.
Let's say they had sets of these plant pods in 20ft x 4ft x 1ft arrangements. You'd only need 10 of such platforms to have enough for a two-man crew. Stack five of them on one side and five on the other, and you'd have a working greenhouse capable of providing oxygen for a two-man crew.
Of course this is no small measure. These sets that we're talking about would be quite large.
In terms of possibility, I'd say it's 100% possible, even with today's technology, to achieve something like this. Perhaps not yet with harvesting water, but I can imagine technical means of doing so. In terms of considering it, I'd say there's an incredibly low chance--it's already wildly expensive to build anything of moderate size to send to space. Accounting for an 800ft3 layout of plants would make no sense.
And clearly, there are a lot of variables to account for. It simply wouldn't be foolproof at this point to send astronauts on a voyage of any distance without a controlled amount of oxygen.