I know that item location but I cant swim into the round tunnel, because its too small for me. What I did wrong ?

Try choosing your smallest character as your lead character. Others report difficulty sometimes with getting through this area - just keep trying to move around , twist, shift whatever. You should be able to get through!

Wyv

Some people reported that switching to graphic software mode could more easily pass the under water tunnel. Try that if Wyv's advice won't help.