I feel as though Oda hammering home the "Oden was meant to boil" thing is way passed foreshadowing at this point. Like it has come up so many times that I don't think Oda is doing it as his usual winks to dedicated fans. I think he deliberately wants everyone to know something bad is coming all the time. This is a bit outside of Oda's style so I think there's two possible reasons why he's doing this:
- He wants to lessen the Shock value of Oden's eventual gruesome death since he will in fact show Oden boiling alive, which is the most hardcore thing that will ever be shown in One Piece. I think Oda wants to mentally prepare readers for Oden's death so that he can get away with Showing something so horrific since very R-rated things usually get censored because the audience is too disturbed when they are caught off guard..... Little history note: People who got executed in this method back in Old Japan were BOILED IN OIL not Water so this is more gruesome than you probably think.
- Oden might be alive and Oda is hammering home his death for an elaborate red herring. I don't like this outcome but it's possible that's why this is being made so forefront.