We reveal how water solvent determines the self-assembly pathway and stability of organic rosette nanotubes (RNTs) and show their possible functions, using three-dimensional molecular theory of solvation (a.k.a. 3D-RISM). Structural water molecules penetrate the pockets on the RNT outer surface, form a wetting monolayer in the RNT channel and bridge RNT rosettes. We predict that the inner water shell might stabilize rare gas atoms inside the RNT channel, and envision molecular devices with RNT channels transporting water or holding guest molecules for targeted delivery.