This work makes use of the ring-polymer instanton method to calculate tunnelling splittings in an isomer of the water decamer called PP2, the enneakaidecamer (19mer) and the icosamer (20mer) clusters. A detailed discussion of the numerical implementation and some computational strategies used throughout the work is given, following a brief summary of the ring-polymer instanton method itself. For the investigation of tunnelling in the PP2 isomer of the water decamer the computationally expensive, but accurate WHBB potential from the Bowman group is used. Due to the symmetry of this system, calculation of a single instanton is enough to predict the tunnelling splitting pattern in the high resolution tetraherz laser vibration-rotation-tunnelling spectrum. Next a comparison of the cheaper TTM3-F potential with two more accurate potentials from the Bowman group, WHBB and HBB2, is made based on instanton calculations in the water decamer and dimer. The TTM3-F values were never out by more than an order of magnitude such that the kink path weights obtained using TTM3-F can be safely used to distinguish important from unimportant tunnelling paths. This becomes especially relevant for larger clusters where calculations using the TTM3-F potential are considerably faster than their equivalents using the Bowman potentials. Finally we consider some examples of instantons in the water 19mer and 20mer where all calculations are done solely using the TTM3-F potential. The rigidity of the clusters and the small resulting kink path weights lead to the conclusion that tunnelling is very unlikely to be experimentally observable for both clusters.