When modelling gravity waves, it is important to note that the upper boundary (water/air) is time-variant. Assuming vortex-free conditions, the problem can be solved with a potential-theoretical approach. In the approximation of linear wave theory, an explicit solution is obtained. In the applications, the refraction near the shore and other phenomena of ocean waves are explained. Furthermore, we discuss Hagen-Poiseuille’s law on laminar pipe flows and its significance in the regulation of blood pressure.