When he is gone a head appears in the water.
Try to talk to it and touch it.
You are drawn into the sea and taken to a strange room where you are able to breathe.

Look at all the drawings on the wall. You learn a lot from them.

To be able to breathe in the water, you are supposed to eat one of the blue polyps on the wall.

To be able to talk to the mer-people, you have to find a black pearl, dip it in a mixture of blood and green stuff to get a golden pearl and eventually eat the golden pearl. 

Only polyps are available in the room
Take a BLUE POLYP from the wall and eat it.

Exit the room and swim to the city.
In the big room try to talk to the creature. You cannot understand the language. Try to take the spear on the wall.
The creature sounds angry and you decide to leave the spear alone.
Look at the GREEN STUFF around the walls. Can it be the green stuff you need? Take some.

Swim back to the bubble and look around. 
Look at the big SHELL outside. Open it and look inside. There is a BLACK PEARL. Take it

Enter the Bubble.
Use the PUSH PIN with yourself to get some blood.
In the inventory combine the GREEN STUFF and the blood. 
Than combine the BLACK PEARL with the mixture and you get a GOLDEN PEARL.
Eat it. Now you should be able to talk with the people here.

Return to the city.
Talk to the creature and you'll learn she is the queen of the maernum-people.
Before leaving take the CRYSTAL from the niche in the wall and show it to the Queen.
You are allowed to take it.

Swim back to the breathing house. To the left of the screen, you see some seaweed.
Look at it, then take it. You will find another CRYSTAL. Pick it up.
Look at and take the seaweed once more. 
Now you find the entrance to a cave. Enter the cave.

On the floor you see two more crystals.
Take them. Look at the altar and you'll get a close up.
There are four blue holes where you shall place the crystals.
They are marked with the symbols, Harpoon , Water, Fish and Mountain.

Look at the crystals. They are marked with the same symbols, three on each.
You have to place the crystals so that the symbols on their sides are pointing towards the same symbols on the altar.

You can do this with trial and error but if you want the full solution here it is. 
Put CRYSTAL 1 on the place marked Mountain and turn it so that Fish points to Fish and Harpoon to Harpoon. 
(The turning symbol is in the mouth's place).
Put CRYSTAL 2 on Water and turn it so that the symbols point to their equivalents. 
Put CRYSTAL 3 on Fish and CRYSTAL 4 on Harpoon and arrange them properly.
Now you have to turn the inner circles. On them you find symbols that are the opposites to the first symbols.
Look at the symbols to get their names.
Turn the circles so that they are placed beside their opposites in the outer circle.
Put Fire beside Water.
Put Bird beside Fish and 
Maernum beside Mountain. 
The pot is already in the right place beside the Harpoon.

The cave is lit with a blue light.
Look at the drawings on the walls. You learn about the history of the Maernum people.
Return to the city and talk to the Maernum queen. She will follow you to the old Temple.
When you are back in the city she gives you the SPEAR and tells you to kill a snapjaw.

Return to the bubble, then swim to the right and you'll reach the shipwreck.
When you try to enter a bid Snapjaw appears. 
Use the SPEAR from the inventory on the SNAPJAW. Wait until you see it blinking, then click. 

When the Snapjaw is dead look at it and take a TOOTH, than enter the shipwreck.
Take the TALISMAN, and return to the queen.
Give her the TOOTH and the TALISMAN.
Talk with her.

Return to the Temple. Look at the wall. You see a symbol similar to the one on your talisman
Use the TALISMAN on that symbol. A niche opens. Look in it and take the STONE.
Look at the stone. It is only one half of the stone you need. Where is the other half?

Return to the queen and give her the stone.
You will be taken to Alais and the chapter ends.