They were towed, they certainly weren't powered, so they were really aircraft barges.
The first true aircraft carrier, with both fly off and landing capabilities and which was a powered ship in its own right was, IIRC, the HMS Argus.
Damn, I had to go look it up...
It was HMS Furious, although it didn't have a flat deck. It had a flight deck ahead of a bunch of superstructure and a flight deck aft on which landings could be made. Argus was the first flat deck carrier.
Apparently the first carrier to serve in war was the Ark Royal, which could launch and recover seaplanes, and could also launch wheeled aircraft, but couldn't recover them.