I found pirate treasure on sea bottom right next to ruins of Colliseum, but cannot find any dungeon. I found one lever, pushed it and it loaded dungeon and I entered some tunnel still full of water, tunnel had no doors and ended with some picture of big octopussy. Only thing I can do here is to drown. Then I tried hall near temple-like ruins, it goes down to some room, but stairs are blocked by sand. Is entrance somewhere around in here or Colliseum has more buildings?

Hmmmm

sounds like the trouble I had, well my friend that you found is the EXIT from Colliseum, and yes there is another entrance its not hard to find, offcourse then you are blurb blurb, its hard to concetrate, the solution to your problem is:Go hunting crabs, make
sure to collect their shells, around 80% off the time they have one, then you have collected 3 shells, go sell them to the harbor in Brimlock, for each 3 shells you sell
you will be able to buy 2 underwater helmets and 2 airtanks, repeat as nessecery,
best hunting ares seems to be just outside Brimlock and northen waterway....

Thanks a lot, I didn't know there are air tanks or chance to get more air helms. I was casting breath of air, but very high lvl spell so I got only couple shots. It seems I have to get some crabs. I wish my ranger got tracking