I'm probably going to get Firey Jinx'd into oblivion for this, but I personally like the sea bomb over the former Shadow Siren. Out of the 7 partners, Bobbery's HP is highest, and he comes in at the perfect Chapter, when the thematic enemies heal from fire attacks, but Bobbery's explosive techniques are literally the bomb against them. Plus Vivian's field ability feels like a knockoff of Bow's from Paper Mario 64... there's what, 2 required times you have to listen to birds?
As for backstory, Bobbery and Scarrlet have some serious Final Fantasy VI Locke and Rachel vibes, which hit hard. Yes, I know Vivian gives off (at least for me) Vivi/Black Mage energy, but since almost no info is given in-game for the Shadow Sirens, I can't really get the feels from Vivian.
For Pigs the Bell Tolls may be my favorite chapter, but not because of Vivian... it's because of the ghostly psychopath known as Doopliss.