Molecular dynamics simulations are employed to investigate the tensile properties of water confined in carbon nanotubes (CNTs) with varying diameters, where CNTs serve as a prototype for xylem vessels critical to long-distance water transport in tall trees. Our results reveal that nanoconfined water exhibits unexpectedly ultrahigh tensile strength (~126 MPa), which is significantly higher than that of glass (~33 MPa). Meanwhile, the tensile strength of confined water is highly dependent on the nanotube diameter. The underlying mechanism for this unique tensile behavior is attributed to the synergistic effect of hydrogen bonding and nanoscale confinement. This work provides key insights into understanding the mechanism of water transport from roots to the canopy in tall trees.