Maybe MIB can't be Smokey when he's under water.
Ben had to unplug the drain underneath his house in order to "release" him.
This is one of those questions in the series which I'm beginning to doubt that the producers will end up explaining.
Why on earth would Ben have EVER wanted to release the MIB/Smokey? Even when it was to save himself and defeat Widmore's people. It still makes no sense whatsoever.
Because, if the Others/Dharma had the ability to control or summon the MIB by using that drain, then doesn't that mean that they had been deliberately releasing him every time he showed up in the first few seasons and started killing people?
Which makes it further strange that the Dharma camp would've needed the Pylons in the first place, if they already had MIB/Smokey contained under water using that drain.