•Sediment thermal properties are highly variable.•Seasonal forcing causes large spatial and temporal sediment temperature variations.•Power cable temperatures strongly depend on thermal properties of a given site.•The 2K-criterion is not suitable to detect man-made heat injection. Analytical and 2D finite-element solutions of the heat transfer equation are used to model the temperature evolution in the upper 15m of marine sediment deposits. These calculations comprise realistic, in-situ measured, thermal properties as well as natural (geothermal and sea bottom temperature) and man-made (e.g. high voltage, subsea power cables) heat sources. A main objective of this article is to focus on the influence of the thermal heterogeneity of sediments in the North and Baltic Seas. In combination with seasonal variations of bottom water temperatures, these result in large temperature differences in the upper 3–5m of the sediments. Also, the impact of additional heat through power cables is considered. The modelling results show maximum cable temperatures as much as 16K difference less compared to a conservative homogeneous scenario previously used from literature values. It can be shown that the 2 K-criterion is not a suitable instrument to monitor the man-made impact to the thermal conditions without knowing the actual thermal settings of a given location.