Glutaraldehyde (GA) cross-linked polyethyleneimine (PEI) coatings have previously been reported to effectively and selectively take up copper from seawater relevant concentrations in artificial seawater. We evaluate the copper uptake of such coatings from natural seawater. X-ray photoelectron spectroscopy elemental analysis revealed the coatings to be highly efficient and equally selective for copper uptake in natural seawater, reaching a maximum copper loading of 2 wt% in 48 hours. Similar to observations in artificial seawater we found that zinc was initially accumulated in the coatings, but was exchanged by copper over time. We investigate the spatial distribution of copper in the coatings by time-of-flight secondary ion mass spectrometry (ToF-SIMS), which revealed that copper was evenly distributed in the coating, with the exception of lower concentrations at the coating-water interface. We use synchrotron X-ray absorption studies and Fourier transform infrared (FTIR) spectroscopy to show that the copper-ligand interaction was mediated by Schiff's bases (imines).