Can I or passengers start the engine after I as the driver has fallen into the water?

Yes. In a man over board situation MOB+ will automatically turn of the engine and then activate Override Mode after 6 seconds.

When in Override Mode those still on board will be able to start the engine directly without any interaction with MOB+. This gives additional safety as those on board do not need to know how to operate the system, but still can rescue the driver quickly.