The room after the boss
Ancient Tomb [7] expands after the Bottled Ship encounter. The upper-left planks lead to a Sword lever. It opens the gates around the lever and the upper-right box. Push that box down-left onto the next trigger, collect the key and unlock the large middle door.
Use the Bottled Ship to leave
The dock in [8] introduces the new field tool. Continue through the water route; the dock in [11] returns you to [8] so you can use the previously inaccessible exit toward Oasis.
Optional secret room
A Shrinking hole in Ancient Tomb [10] leads to Ancient Tomb Secret [1], the room on a large statue’s hand. This is a detour; it is not an extra Great Flocci phase or the chapter finale.