... what if it wasn't a cannon ball? galileo could've dropped any type of ball, like a leather football. It wouldn't have broken the deck because it wasn't iron...
Lots of these stories aren't entirely accurate. Galileo didn't prove all weights fall at the same speed from the leaning tower of Pisa either... the results there were inconclusive, he had to do it another way.
Also, imagine nerd Galileo carrying a beeeg, heavy cannball while climbing the mast...
even the sailors would have a hard time going up with both hands tied.