User blog comment:Pacifista15/Seriously, Luffy?/@comment-4498721-20111123074749

He tried to destroy a gigantic ship with Elephant Gattling, probably it caused too much stress to handle with his wounds from the fight with Hody, also he was underwater.

Luffy has improved as much as the other SHs but his new skills were obvious, his control over haki/gear was expected, and that's pretty much everything he learned in 2 years.