This book aims at the application of Riccati equation, which widely exists in the marine motions, and is important for the motions' vertical structures. Although this equation is a nonlinear differential equation, but it can be reduced from a second order linear differential equation, and there are few oceanographic papers involve this equation. In this book, we will present the mathematic structures of the Ekman spiral under the assumption of non-constant eddy viscosity with application of this equation, in addition, on the basis of the Ekman theory and geostrophic approximation, we will give a relationship between the function of northward pressure gradient and the Equatorial Undercurrent.