Wave tank tests facilitate the understanding of how complex sea conditions influence the dynamics of man-made structures. If a potential deployment location is known, site data can be used to improve the relevance and realism of the test conditions, thus helping de-risk device development. Generally this data is difficult to obtain and even if available is used simplistically due to established practices and limitations of test facilities. In this work four years of buoy data from the European Marine Energy Centre is characterised and simulated at the FloWave Ocean Energy Research Facility; a circular combined wave-current test tank. Particular emphasis is placed on the characterisation and validation processes, aiming to preserve spectral and directional complexity of the site, whilst proving that the defined representative conditions can be effectively created. When creating representative site-specific sea states, particular focus is given to the application of clustering algorithms, which enable the entire spectral (frequency or directional) form to be considered in the characterisation process. This enables the true complex nature of the site to be considered in the data reduction process. Prior to generating and measuring the resulting sea states, issues with scaling are explored, the facility itself is characterised, and emphasis is placed on developing measurement strategies for the validation of directional spectra. Wave gauge arrays are designed and used to characterise various elements of the FloWave tank, including reflections, spatio-temporal variability and wave shape. A new method for directional spectrum reconstruction (SPAIR) is also developed, enabling more effective measurement and validation of the resulting directional sea states. Through comparison with other characterisation methods, inherent method-induced trade-offs are understood, and it is found that there is no absolute favourable approach, necessitating an application specific procedure. Despite this, a useful set of ‘generic’ sea states are created for the simulation of both production and extreme conditions. For sea state measurement, the SPAIR method is proven to be significantly more effective than current approaches, reducing errors and introducing additional capability. This method is used in combination with a directional wave gauge array to effectively measure, correct, and validate the resulting directional wave conditions. It is also demonstrated that site-specific wave-current scenarios can be effectively re-created, thus demonstrating that truly complex ocean conditions can be simulated at FloWave. This ability, along with the considered characterisation approach used, means that representative site-specific sea states can be simulated with confidence, increasing the realism of the test environment and helping de-risk device development.