No one is going to confiscate a multi-usb charger. Even the "high power" ones that charge 6 devices draw less than 1 amp, which is waaaaaay too low to worry about. For comparison, a hair dryer might draw 10 amps or more.
The only things that get confiscated are power strips that have multiple 110v power sockets on them, and 110v power extension cords. Some of them are potential fire hazards, and rather than train people on exactly which ones are OK and which aren't, they typically just take anything with multiple outlets or with a single 110v plug on it and a cord. Of course, they don't always catch everything, but if they do see a power strip, chances are good they're going to take it until the end of the voyage.
I have heard that if you insist, they'll ask a ship electrician to inspect your cord or power strip (which will happen at some point on the first day; they confiscate it until it's inspected), and if it's acceptable they'll let you use it. IMO, it's easier to just use one of the multi-outlet extension cords you can borrow from guest services.