I'd pack it - the less you have going through the security check point the better.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014, 2016, Royal 2017
Star Clippers: Royal Clipper 2015
If I pack an extension cord in my luggage will it go through the baggage check okay? I'm picturing them thinking it's a bomb or something. Or should I put it in my carry on bag?
Pack it in your checked luggage and be sure it's in excellent shape. Only use it in your cabin when you are there. When you're done recharging, unplug it and put it away. (Some Room Steward's on Carnival have been told to "confiscate" any extension cords, no matter what condition they're in .)
While I'm all for safety, I'm afraid they've gone "overboard" on this!
"A truly happy person is one who can enjoy the scenery on a detour."