Inspired by a critique of World's Largest Dungeon that mentioned a black pudding with extra HD.
I saw in my mind a vast underground lake. However, the water was actually a colossal black pudding. The party arrived at its shore and spotted a large, flat boat carved from solid stone. Such a boat would never float on water but stone is the only material that can withstand the powerful acids exuded by a black pudding.
The black pudding is a viscous ooze, so the boat cannot just float across it like on water. No wind will sail the vessel in the sticky stuff. No oars can propel through the muck. No pole can find purchase in the lakebed far below. Instead, the party has to manipulate the black pudding into moving the boat by tossing wooden or metal objects into the slime behind them. The black pudding reacts to such "food" by swelling and engulfing the prize, thus shunting the boat a short distance in the opposite direction.
Of course, that does mean the party must find a way to move the incredibly heavy boat onto the pudding in the first place, and far enough from shore to accommodate its strange propulsion.
