  Narrow resonances decaying into $WW$, $WZ$ or $ZZ$ boson pairs are searched
for in 36.7 fb $^{-1}$ of proton-proton collision data at a centre-of-mass
energy of $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the Large
Hadron Collider in 2015 and 2016. The diboson system is reconstructed using
pairs of large-radius jets with high transverse momentum and tagged as
compatible with the hadronic decay of high-momentum $W$ or $Z$ bosons, using
jet mass and substructure properties. The search is sensitive to diboson
resonances with masses in the range 1.2-5.0 TeV. No significant excess is
observed in any signal region. Exclusion limits are set at the 95% confidence
level on the production cross section times branching ratio to dibosons for a
range of theories beyond the Standard Model. Model-dependent lower limits on
the mass of new gauge bosons are set, with the highest limit set at 3.5 TeV in
the context of mass-degenerate resonances that couple predominantly to bosons.
