In this article, we present a detailed mathematical analysis of the phenomenon of water waves guided by the coast. We work with the mathematical model issued from the linearized theory of gravity waves. Our results mainly concern the existence, the number and the high and low frequency behaviours of the guided modes. These results point out a large variety of phenomena which are deeply influenced by the geometry of the coast. We obtain them with the help of the spectral theory of unbounded selfadjoint operators. Some numerical results are presented at the end of the paper to illustrate the theory.