Looking at Megatron's left arm, I'm thinking he took up fishing while being underwater for all that time. Not a bad way to pass the time I guess.

Either that or it's a reference to Captain Hook. Peter Pan Optimus Prime repaint in the future perhaps?

OR it's giving us a hint to the new cartoon after Animated...Pirateformers! "Yarr, transform 'n' roll out the barrel yee scurvy dogs!"