The Arctic hydrological cycle undergoes rapid and pronounced changes, including alterations in oceanic and atmospheric circulations, and precipitation patterns. Stable water isotopes (δ18O, δ2H, d-excess) can be used to trace these processes including their potential to feedback into the global climate system. The MOSAiC expedition provided a unique opportunity to collect, analyze, and synthesize discrete samples of the different hydrological compartments in the central Arctic, comprising sea ice, seawater, snow, and melt ponds. Here, we present spatio-temporal variations in the isotopic signatures of more than 1,000 water samples. We found that (i) average seawater δ18O of -1.7‰ conforms to observed and modelled isotopic traits of the Arctic Ocean; (ii) second year ice is relatively depleted compared to first year ice with average δ18O values of -3.1‰ and -0.7‰, respectively. This might be due to post-depositional exchange processes with snow, which has the most depleted isotopic signature among all compartments (mean δ18O=-15.1‰). Our dataset provides an unprecedented description of the present-day isotopic composition of the Arctic water covering a complete seasonal cycle. This will ultimately contribute to resolve the linkages between sea ice, ocean, and atmosphere during critical transitions from frozen ocean to open water conditions.