Umi to Mizugi to Wooser to (S01E06)

Wooser saves a turtle at the beach, and that turtle leads him to an underwater world... the next day he is found drowned. The mysteries deepen as the case twists and turns. A trick with the time tables, and the shadow of the true culprit lurking in the background... Who is "The Animal Which must not be named?" A shocking truth is about to be revealed... or if it is, that would be really cool, anyway. (Crunchyroll)