In recent years there has been a recent interest in implementing the applications of terrestrial systems but in underwater communications systems (UWS). These systems presents technical challenges mainly due to the composition of the water and the phenomena that occur only in these environments. In order to improve the performance of UWS, techniques that have been proposed for wireless terrestrial communications systems must be studied but considering the characteristics of the underwater channel. Thus, this paper presents closed-form expressions that allows to evaluate the spectral efficiency (in terms of out-of-band emissions) of UWS using the Generalized Frequency Division Multiplexing (GFDM) waveform. These closed-form expressions are used to obtain numerical results for many different parameters system, allowing a Quantitative evaluation of the spectral performance.