 The study of animal movement and use of space have traditionally focused on horizontal and vertical movements separately. However, this may limit the interpretation of results of such behaviours in a three-dimensional environment. Here we use passive acoustic telemetry to visualise and define the three-dimensional use of space by two species of sea snake [Hydrophis (Lapemis) curtus; and Hydrophis elegans] within a coastal embayment and identify changes in how they use space over a diel cycle.