Because the stereotypical Slytherin wasn't content with simply valuing bloodline and exhibiting elitism in this regard, the Slytherin common room sits underneath the lake. Some call it overcompensating, others call it cool. Either way that's where it is. The entrance itself is in the dungeons, disguising itself as an empty cell until the correct password is said. Get it right the first time. They're not known for their helpful nature after all.