eDNA in the marine environment is much more dilute than in freshwater systems and so the detection probability for any species in a given sample will be lower (note other survey methods are also less sensitive in the open ocean), and means it is important to filter more water and collect a greater number of samples in the marine environment.
Generally, the more samples you can collect, the more representative and comprehensive your dataset will be. At the moment it’s very difficult to say how many samples are needed for a comprehensive survey in the ocean, or what depths these should be collected from, and spatial interpretation is also difficult because of the complexity of currents and other aspects of oceanography - there is definitely the opportunity for lots of large-scale research here!
However, eDNA does still provide a lot of data in the marine environment and compares very favourably with alternative tools in this regard. In one pilot study we did in the North Sea, just three eDNA samples detected ⅔ of the species that had been recorded in a 2-year netting survey that had cost £150k.