Rafts can be used to travel across the witch water seas of the Spooky Dimension. You can craft them from either bones from the skeletal trees or wood from the spooky forests.

Sounds to me like we need a dungeon that only spawns in witch water seas, with the entrance barely peeking out on top of the water's surface. Would provide a good reason to boat around (aside from traveling to island biomes).

Maybe Spookium Armour could be added that stops the negative effects of Witch Water?