In ship model tests, a model-ship correlation line (e.g., the ITTC57 formula) is used to calculate the frictional resistance of both the ship and its scaled model. However, this line is designed for deep water and the effects of water depth is not considered. Research has been conducted to improve the correlation line in shallow water, but studies of the extremely shallow water case (depth/draft, h/T < 1.2) are rare. This study focuses on the friction of two ship types in extremely shallow water, where the ship’s boundary layer cannot develop freely. The physical details are analyzed based on the data generated with Computational Fluid Dynamics (CFD) calculations. The results show that for certain ship types at the same Reynolds number, the frictional resistance becomes smaller when the water is shallower. The geometry of the ship, in addition to the Reynolds number, becomes essential to the prediction of ship’s friction in extremely shallow water. Therefore, this scenario is different from intermediate shallow and deep water, and the prediction method should be considered separately. The data and analysis shown in this study can help to improve the understanding and prediction of ship’s frictional resistance in extremely shallow water.