I've enjoyed several volumes in this wacky series, starting with Volume 1, and decided to pick up this volume after spotting it at Barnes and Noble today.
It continues the 4-panel-strip humor about high school student Tatsumi and his apparently-permanent roommate Wakasa, a merman who has taken up residence in Tatsumi's bathtub, along with an assortment of other mer-people who wander in. High points here include:
Agari, the shark-man, has tooth problems - a major issue for a shark; Tatsumi gathers some heavy equipment to try and help, only to find that even Agari is scared of the dentist!
Wakasa and Tatsumi switch bodies, leading to Wakasa's discovery of school life (once he's figured out how to use the legs on his new body), while Tatsumi experiences a watery life.
Takasu the octopus-man falls ill - he changed colors to blend in to his surroundings and then couldn't change back, so the gang tries to find ways to re-tint him.
Funaki, an agent from the "unknown creatures countermeasures" agency, turns up intending to study Wakasa - seems he really wants to have the merman swimming "freely" in a huge aquarium in Funaki's own place. The gang is not up for this, but it did add a slightly more sinister element to what's usually a very whimsical story.
Oh, and there's a nice surprise on the very last page. Tatsumi's school-friend Sousuke got very little page-time here, so on that last page there's a full-page illustration of him - in sexy-body-pillow style, shirt open, pants sitting a bit low, sexy smile on his face!
[There's a TV Tropes page on the series. There's an anime as well, though it doesn't seem to have been dubbed in English yet.]]