In this paper the thermal behaviour of both a shallow and well-mixed layer of water and its underlying sediment (e.g. a lagoon) is described. The water-sediment thermal interaction is analytically obtained by assuming a sinusoidal trend for the water depth. During the first half-period, the water leaves the lagoon at temperatureT(t) and in the second half-period the water comes back but at a constant temperatureT ^*. In this study theoretical results in a case without the solar flux will be shown (i.e. the case correponds to a typical winter case).