I went left around the electrocute water but didn't see any keys, and already blew up the shutter. It is the room after the one with the shutter where the line of blocks are in the way. Is the Boss key hidden? Sorry to ask so many questions, but I'm having a hard time figuring the maps out. I thought I had to get past the blocks to get a key. Isn't that the yellow dot on the map? Up from the block room and left?
Up from the block room and left is indeed a key. You can bomb those blocks. And you didn't see the boss key? Well, when you go down, down,left, you immediately see a big chest. In this quest, most boss keys are in big chests. (Just like items.) Just walk into the big chest to open it and then again into it to get the key.
EDIT:
Jako9b:
Only the dungeons that are ouside of Hyrule (starting with IW L1) have subdungeons, so if you are still at Source of the Flow, there's no subdungeon. If you are indeed at IW L1, look through the main room to find a strange part of the ground and use the dungeon item on it.


