This is where Sanji in Nami's body went to the lake to get the icy water after Smiley, the slime that Casear sealed away, killing fish and everyone else to find the samurai's torso. Zoro and Brook were waiting for Sanji to rescue the samurai's torso from the shark. Sanji then used Haki to locate the body and realized it is in the shark's mouth. He managed to retrieve the torso and to escape before the poisonous gas consumed him. The samurai then reveals himself as Kin'emon from Wano. 
GIFs
