When you get to the save point and go to the room where the Blood Eels appear, go back to land, and take out the Eclipse Scythe. This strategy works much better than using the Harpoon Gun because the eels cannot damage you while you are on land. Keep attacking the Blood Eels until they are all dead. Once the eels are dead, go back to the save point. Save the game, and the eels will automatically respawn each time you go back into the room. This makes collecting Yellow Essence easy as long as you stay on land to kill them.