How do you find the 2nd sea chart on the phantom hourglass

After defeating the 1st boss Lineback tells you to go to Mercay Island then go to the Ocean King Temple there Oshus will tell you about the phantom hourglass. U get the hourglass and u can go father in the temple. there is a door you will need the first spirit to open it. I think you can take it from there



ya but how do do you get past the second floor B2?