Automated aesthetic assessment of photographs is an active research area with applications in image editing and retrieval.  There are many suggestions on the various factors of importance in making an image 'good' or 'aesthetically pleasing'.  However, in the literature there is no consensus on a definitive set of attributes that contribute to image aesthetics for both terrestrial and underwater photographs.  Whilst underwater image aesthetics shares some common ground with terrestrial image aesthetics, due to the differences in lighting, environment, subjects, and various aquatic features there may be different factors which contribute towards the definition of a good image.  In this research we interview underwater photographers and apply thematic analysis to their responses with the aim of determining which attributes are important for an aesthetically-pleasing underwater image.  The results define a set of nine key attributes (i.e. Aesthetics, Aquatic features, Colour, Composition, Image precision, Lighting, Novelty, Subject(s), and Technical competence) which share similarities and differences to those suggested for terrestrial photographs.  These findings will guide future work in automated assessment of underwater image aesthetics.