The disclosure relates to a well cooler (3) for cooling a cooling medium of a driving unit on a ship (1), such as a marine motor, by means of outboard water (8), which well cooler (3) can be placed in a well space (2) present in the (1), which lies at least partially below the outboard water level and which in open communication with the outboard water (8), the well cooler (3) comprising at least one cooling element (4) for the cooling medium, which extends into the well space (2) and which is surrounded by outboard water (8) during operation, and the ship (1) furthermore being provided with a cathodic corrosion protection system (10). The object of the disclosure is to provide a well cooler (3) of the above kind which on the one hand exhibits and retains an effective cooling efficiency, and which on the other hand is not affected by stray current corrosion, which reduces the life span of the well cooler. According to the disclosure, the well cooler (3) is provided with means (14) that prevents the cooling element (4) from being affected by stray current corrosion.