InvestiGators (#1) by John Patrick Green

From First Second Books:

John Patrick Green's goofy graphic novel series follows the super spy alligator duo the InvestiGators as they travel through the sewers and fight the forces of evil.

MANGO and BRASH are the INVESTIGATORS:
sewer-loving agents of S.U.I.T.* and scourge of supervillains everywhere!

With their Very Exciting Spy Technology and their tried-and-true, toilet-based travel techniques, the InvestiGators are undercover and on the case! And on their first mission together, they have not one but two mysteries to solve! Can Mango and Brash uncover the clues, crack their cases, and corral the crooks—or will the criminals wriggle out of their grasp?