The activity of rowing is majorly performed outside, namely on rivers or lakes, and the probability of getting the t-shirt wet due to splash or rain is high. In order to evaluate the impact of water repellent finishing on the thermal insulation, several rowing shirts made with different structures and raw material were tested and compared by means of a thermal manikin, before and after finishing. The shirts were treated with a 5% of a fluorocarbon-based product water repellent finishing. The heat loss values of rowing shirts were measured by using a thermal manikin and the effective clothing insulation values were calculated. Moreover, the effect of garment design on thermal insulation was investigated by comparing shirts with more than one knitted structure with single knitting structures shirts. Considering the results for the heat loss values, after WRF, the heat flux of rowing shirts was lower. Shirt B and shirt B-1 had higher effective thermal insulation value after WRF, where the other shirts had lower thermal insulation values. Moreover, not only the water repellent finishing, but also the structure of garment is also important in terms of heat loss and thermal insulation.