The Umi-bozu is located on the The Ocean Roars Again Mission. These yokai of Japanese Folklore are depicted as extremely large, black humanoid creatures that appear in the oceans, often only showing their heads and shoulders. Larger than some buildings, they appear on otherwise calm seas, during the night, to destroy ships. The strategy for this fight is buff your weapons with fire, get in close and be agressive. You’ll want to hold block when you are unsure what he is going to do next or dodge to the side if you do. Be sure to reapply the fire buff as necessary. In phase 2 he will move the the middle of the dock and have 2 adds. Ignore the adds and focus on the boss. When he jumps into the air, dodge, then move in for the attack. Be sure to avoid the AoE he does shortly after landing. .
As found on Youtube