Actually, in order to get past the 8th floor, you need to stop the water current first (and I find a well thingy on the sixth floor but it wouldn't let me to stop those bubbles from coming out. If you check the map on the 7th floor and the 8th floor, you will find out that there is actually only one stair that really connects to the 8th floor, and you will find out that there is a water current on the 7th floor that blocks you from getting through (should be somewhere on the bottom left side of the map).
Thanks for your help anyway. But what should I do?