Areus nods to Callaphe "Yes, let's try to explore carefully the room first." He swims near the statue and paying attention to not touch it he tries to take a better look at it (Perception 18).
He will then swims towards the hero. "Check carefully your room Hero!" says Areus.

The spear looks to be a regular spear, large enough to be in proportion to the gigantic statue.  The statue's chest is taller than Areus making it very tall.  The statue is likely 25 feet or more in height, the spear the same length.

The hero swims around the room and tries to pull on the door and it does not open.  

Areus and Callaphe are inspecting the statue together when they see faint lines in the chest and believe there may be some sort of door in the chest of the statue.  However, they don't see any means to open the door in the chest of the statue. 

You do see that when the hero pulls on the chain a door in the chest of the statue opens up.

Areus says to the Hero "There is some kind of door in the chest of the statue, try to check it on your side too!"

Saya will help pull the chain when the party is ready. “Ready when you guys are.”

"Let's wait to see if the Hero manages to open the door on his side." replies Areus "If he does not manage to open it, we can pull together on this side while he pulls it on the other. Since the murals are somehow linked to the actual rooms, maybe this time we have to do the same thing at the same moment."

Callaphe points at the hero and the chain he pulled on. "That seems like a pretty obvious clue."

"You are right Callaphe, all this water is not making me see clearly." replies Areus "Shall we pull it all together then?" The hoplite says to the group and the hero.

"I don't quite like the look of that chain. Let me pull it, you guys stay out of the way in case it is a trap"

Quote from GoodBovine >>

"I don't quite like the look of that chain. Let me pull it, you guys stay out of the way in case it is a trap"

Saya pulls out her shield and stays behind Soren, in case anything falls from the ceiling. (Though, if we are underwater, things should fall down slower...)

With the hero pulling the chain in the mural and Soren pulling the chain in the room the door starts to open.   The others also see that a door on the chest of the statue also opens. 

At the same time the statue starts to speak. 

“Tsehc desolc a ni ytefas dnif ylno uoy! Regnad slleps tsehc nepo na.”

"C'mon Soren put your kracken into it!" Tisto calls out from Saya's back giving Soren Bardic inspiration.

“Tsehc desolc a ni ytefas dnif ylno uoy! Regnad slleps tsehc nepo na.”

Does Tisto recognise the language (History = 11)

((Is there a check we need to make for this?))

"What kind of a language is just english backwards? It says....an open chest spells danger.....you will only find safety in a closed chest. If that is mean't to be mysterious, it is far underwhelming" 

"Unless it's a warning..." Callaphe says, looking at the mural.

Soren nods "Stay away from open chests"

"Like the one that's open right now?" 

As you figure out what the statue said looking inside the open chest of the statue you see a 10-foot-square compartment.   Inside, you find a chest.

So there is a chest in the compartment that opened up in the chest of the statue.

“I offer to go in and open the chest, if everyone agrees. “ Saya makes her way to the entrance of the statue’s chest.

"Hold on," Callaphe says, "maybe we shouldn't. After all, the statues warning..." she stops and thinks for a moment, "why would the warning be backwards?"