Blockage effects are currently not accounted for in cavitation analyses of tidal turbine rotors. At higher blockage ratios, rotors are more heavily loaded and have potentially stronger suction peaks, so cavitation inception is more likely. In this paper, blade resolved computations are used to carry out a cavitation analysis over a range of blockage ratios and tip-speed-ratios. Our analysis suggests that increasing the blockage ratio from 0.01 to 0.197 reduces the minimum static pressure head in the fluid by approximately 0.5 m. To mitigate this reduction, either the submersion depth of the rotor can be increased or the maximum permissible tip speed ratio reduced. However, reducing the maximum permissible tip speed ratio is shown to severely restrict the rotor thrust and power. Spanwise flow effects are shown to reduce the strength of the suction peak on the outboard blade sections, reducing the likelihood of cavitation inception. Blade element based methods are shown to inadequately account for spanwise flow effects and thus are overly-conservative. Hence, rotors designed with these methods could potentially be operated at higher tip-speed-ratios or reduced submersion depths.