My engine doesn't stop if I submerge the Fob in water or walk away from my boat.

First, If you press the fob and your engine shuts off, the MOB+ is working properly.

Submerging the FOB in a smaller volume of water (bucket or live well) does not have sufficient volume to dissipate the signal between the Fob and the Hub. 

When walking away from the boat you may experience signal bounce off surrounding buildings or vegetation. When on open water the signal between the fob and hub will dissipate and result in engine shutoff at approximately 50Ft