I have not physically seen a mp40. But I think the dry side contains some kind of coil or magnetic driver for the pump!
Saltwater will eats electronic parts in time. As an electrical/electronic engineer and also used to be a cellular and pager repair technician, a proper way is to take it apart and dry it with electronic board cleaner spray. But I would NOT recommend you taking it apart. So, If you have a Computer dust cleaner spray available, spray it with the cleaner and let it dry for a couple of days. If not, use a hair dryer, set it to COOL and try to dry it out and then let it sit a few day to completely dry. Also, try the rice method (I never done it before) but maybe the rice will absorbs the moisture. Never hurt to try. Most important, when you are ready to plug it in, make sure you plug it into a GFCI outlet just in case there still some water left in the unit. Good Luck and give us a update.
Update:
Oh! after searching the Internet, I found some pics of the internal dry side for the mp40. It's a bunch of coils. So I think you should be fine after it dry out.