These fey actually do have a LA already, but it's a ridiculously high +5. For comparison, these things only have 4 RHD, so it doesn't take much to see how it could be lowered a bit.
Ability scores (especially dexterity) are surprisingly high, and a 30 ft. land and swim speed are more than I expected. Even more impressive is the 60 ft. burrow speed, which applies to all movement through anything more viscous than clear water. DR 5/cold iron makes up for the awful HD, and a few immunities (acid, paralysis, poison and sleep) round everything off.
The chassis is decent, but what about the special abilities?
Sicken adds a moderate debuff to their two claw attacks, which is completely insignificant as I see no reason to use these claws at all.
Stagnate spoils all nonmagical liquids that come near you, which is nice for your Liquid Burrow ability, moderately useful anytime you'd otherwise use ingested poisons, and very annoying for allies most of the time. Especially in dungeons, where maneuverability is low, keeping the party's water supply away from their bog imp is difficult.
Finally, there's Sink (alliteration, yay!), which all bog imps get a free Ability Focus for. Sadly, it's quite awful.
Not only does it take several turns for the ability to become effective (1 + target's strength modifier, so it'll only increase as the game processes), it's also kind of weak. Even if the ability prevents the target from taking actions (something implied to not be the case), it's still nothing more than an easily resisted save-or-lose that only works in a handful of terrains.
All things considered, bog imps (like most of the monsters in this book) are far less viable as PCs than as monsters. +0 is justified, however, given their decent chassis and low-ish RHD.