One side, is supposedly there to provide information, and help combat the threat of the warp with what knowledge they've accrued through millenia and they're simply being ignored.
The water guns, are a metaphor for the right tool to fight the fire of the warp. Smaller in scale vastly than what is needed to quench the inferno, but a few hundred thousand stem the tide.
The space lasers in this case is the Culture's own lack of understanding of metaphysics, and how the warp itself is tied to human emotion. Meaning meat sacks are in fact in possession of something that the Minds aren't.
And I wonder how a mind would take that?