Much like the bug corrected in 1.6.2 with the signs not updating water, the player and mob heads have the same problem.
MC-24043
Heads do not update water
Resolved
Duplicate
1
Luís Gonçalves
(Unassigned)
Confirmed
block, head, heads, water
Minecraft 1.6.2
Retrieved
Confirmed, heads are supposed to be destroyed by water, but water won't update.