Hide underwater near a bridge or a secluded place. When somebody comes by, you stealthily jump out and rob the person, then jump back underwater.
Or, if you were Arlong, accelerate out and yell SHARK ON DAAAAAAAARTTTTSSSSS!!!!
However, let's say somebody survives the mugging and tells the authorities, and they find where you are.
You then have a choice; hide underwater until they get bored or go away.
OR
Drown the authorities as they come towards you. That way, you'll get a load of stuff, AND become a famous water-breathing outlaw! EPIC WIN!