Rogue Galaxy Quarry: Raft to the Underworld
The closest save to teleport to the Raft to the Underworld is the Nalvasa Seaway - Endpoint. From here you'll want to head across the grass to the beach on the opposite end of the save teleporter. Right where I am on the map. This is where you'll find the ? circle to summon the Raft to the Underworld.
The item required to summon the Raft to the Underworld is a Dark Music Box. Once you're past chapter 8 or so you'll get these fairly regularly from battles.
If you are missing the item required to summon this boss grind some battles and cross your fingers or go treasure chest hunting on a planet you've 100% the map on. In Rogue Galaxy once you pass a certain chapter the loot table of items that can drop for you ingame is updated.
What I mean is, every item has a chance of dropping off every creature after you've passed that chapter that allows that item to drop. At least this is how it works in my experience. Therefore, if you're missing any items to summon any of the quarry bosses all you need to do is grind a little until you get that item to drop for you. Good luck!