I am stuck , I am outside the sunken city of collasium. I enter a hole
near two chests, pull a lever, then I m in a dungeon consisting only
of a U shaped corridor and a door at the end which never open. It
looks like I am missing something but I don't know what. I have a shell
helm and breathing conque for each member of my party.

Anyone have an advice ?

Swim around some more and look for other entrances. I believe I found 3 in all. The one you are at was actually the last one I located and I know that there are locked doors on the other side which explains why you get no further from where you are.
Guessing here - I think you need to look further East and maybe a little North for the other entrances.

Wyv

I found exactly 3 building udner water, after
going left and right / up and down in the water
area on the map. They are all in the souther part
of the second sea roughly at the same place.

* a building south west with no door in, no other visible exit
* a temple (or temple like building) in the middle/north
with no visible entrance
* last to the south east in a hole with lot of
column around an entrance like other dungeon entrance
it has a lever but only a locked door at the end of the corridor
and absolutly no other passage/entrance. It is locked I can access it.

Is there another vuilding I did not see ??

rough map centered in the southern part of the second
sea and south of an island :


B

C
A


A = hole with no visible issue in a quadrate room
B = temple without entrance
C = where the locked door I cant open is

..........
.....:B....
............
.........C
...A.......

There is a small "corridor" at the back of the temple, but it leeds nowhere... I also wonder about the entrance, there are people who've gotten in, must be a simple mistake?

Musashi

Use the entrance under the temple. It's quite well hidden but is close to the base of one corner. The big entrance near to the chests is actually the exit. The door can only be opened from the inside.