So, for those interested:
I looked it up on the Interweb and is seems Frigoboat (actual company name is Veco S.p.A.) has this system where they use a special developed circulation pump designed to run 24/7. Apparently they did not want to develop a 24 volt version so they only have a 12 volt version. To accommodate a 24 volt ship's system they have some devices (like the pump switch) to lower the voltage from 24 to 12 volt. The compressor and such still runs on 24 volt as this is the standard Danfoss system that can accept both 12 and 24 volt.
So the switch (http://www.penguinfrigo.co.uk/shop/product/581/) itself converts the 24 volt into 12 volt to supply the pump.
The tricky part is that this pump has a fairly low consumption (1Amp. @ 12 Volt) and this pump should not be replaced by just any other pump, given it's duty cycle and noise level.
I'm actually wondering if this is not just a 24 volt pump set to rum at 12 volt to make it last longer and more quiet.
Amel 54's come equipped with a spare pump right next to the active one. There is a reason for this.