Newtralwerowner: Firstly, I would like to confirm you actually have the MS400 vs the 390. To my knowledge, the 400 was first made in 2004, but some 390's also seem to also have the 2004 build date. I think MS shifted over from the 390 to the 400 in 2004.
I have not seen anyone else with a 2003 MS400, but I suppose it's possible. I just want to confirm it so we can make sure we are answering your questions for the correct model, as the 390 and 400 have significant differences. You can verify the year by looking at the HIN number on the transom. I believe the last two digits indicate the year. Others may chime in if I am wrong about this.
Having said that, if you really have the 400 I probably have the answer to your question. That switch is for the raw water washdown pump which was an option on this model. I have the switch too, but not the pump. Take a look in the anchor locker for a spigot or water hose connection of some kind. If you don't see one, that means your boat wasn't built with that option.
Ignore this answer if you actually have a 2003 MS390, as I am unfamiliar with that model!