Environmental DNA (eDNA) based biosensing has significant potential and offers many advantages over current methodologies for marine species monitoring. It is now possible to isolate a given species’ DNA from water samples and use this as a proxy for their detection. While there are many challenges to be addressed in terms of relating DNA based data back to actual species biomass, the sensitivity, specificity and potential to transition to on-site testing is driving innovation in this area. We describe the current advances in environmental DNA based testing with a focus on the three steps or challenges that are being considered for on-site monitoring: 1. DNA acquisition, 2. Molecular Assay development, and 3. Detection mode. We describe the evolution of methods within each of these areas, and highlight which techniques hold the most promise to realise a vision of eDNA based marine biosensing. We conclude with an outline of recent innovations in eDNA based biosensor devices including portable, remote and autonomous systems that, with further development, will facilitate the collection of real-time data on any species of interest.