If you place water to completely flood the floor area around a monster spawner, will it prevent monsters from spawning? Or can monsters still spawn in water? The wiki says:
It tries 4 times to place the mob at a random position in an 9 x 9 x 9 area, and gives up on placing that mob if it cannot find a valid position after the 4 attempts.
So is a block with water flowing over it valid? I'm asking both from the perspective of neutralizing a dungeon (making it safe) and from designing a dungeon trap (a floor with water flowing over it would direct mobs to the trap, but would it also prevent them from spawning in the first place).
Mobs spawn in this gigantic tower...
...and hit these fences. The water then funnels the items into the middle (which then connects to another stream flowing the lava you can see, to kill any mobs that fell through the middle).
Best Answer
The rules for "normal" spawns do not seem to apply fully to Mob Spawners. A dungeon with a completely flooded floor is not safe. Apparently the random block in the 9x9x9 cube can be air.
Here's a flooded dungeon, and here's the same dungeon after I turned the difficulty to Easy (and removed the torch). The spider spawned in the water and climbed up to the ceiling.
Edit: Test repeated with source blocks, and a torch for better visibility.
Setup
Result: still not safe