Just of out curiosity, do we ever get to see the same sea monster that ate Shanks's arm way back in the beginning of One Piece?
I have only kept up with the series until Impel Down, so please no spoilers beyond a "yes/no they did/did not re-encounter the sea monster"
Yes, Luffy did. It isn't shown in the anime, but Luffy reencounters this Sea King, the Lord of the Coast, later in chapter one of the manga. Immediately after setting out from Foosha village Luffy is attacked by the Sea King, and is pleased to have a chance to show it what he has learned in the ten years since their last encounter.
As Qiri stated, Luffy does encounter the Lord of the Coast again. This part actually is animated during a flash back. You're not there yet if you're only at the impel down arc.
This happens at the end of the episode 504 at around 20:40 when he is first setting off on his journey.
When he encounters it he says:
Luffy: You don't know who you are messing with. Look at this technique I've been perfecting for 10 years!
He then hits him with his Gomu Gomu no Pistol just like he does in the manga chapter.
The One Piece Wikia notes that while this scene of Luffy knocking the wasn't depicted in the original backstory segment of the anime, you can see Luffy hitting the Lord of the Coast with his Gomu Gomu no Pistol in the very first episode of the anime, during the original opening song, We Are.
You can see a YouTube video of the first opening here:
https://www.youtube.com/watch?v=8mgdyZQ_wzA
The scene is replayed in episode 737 at 22:10.
