This paper presents a novel Deep Neural Network aimed at fast and robust visual loop detection targeted to underwater images. In order to help the proposed network to learn the features that define loop closings, a global image descriptor built upon clusters of local SIFT descriptors is proposed. Also, a method allowing unsupervised training is presented, eliminating the need for a hand-labelled ground truth. Once trained, the Neural Network builds two descriptors of an image that can be easily compared to other image descriptors to ascertain if they close a loop or not. The experimental results, performed using real data gathered in coastal areas of Mallorca (Spain), show the validity of our proposal and favorably compares it to previously existing methods.