A week ago I was swimming in the sea and abruptly I felt water flowing into my left ear. It caused pain that later increased. I tried to get the water come out of the ear (jumping on one foot etc), but nothing helped. Since then I keep feeling this water in my left ear.
Yesterday I went to an ear doctor (ENT) who just looked into my left ear and told me I had an inflammation there and prescribed antibitotic. I asked her whether there was still water in my ear. No water, she said.

But I still feel that something (water?) is behind my eardrum and I don't understand how the doctor could see what is behind the eardrum. I am afraid if there is water there, the inflammation can't be healed and it will only worsen.

My question is: Is it possible that water flows behind the eardrum into the middle ear and it stays there days long and do not come out (e.g. through the Eustachian-tube)?

The ENT has something called an Otoscope (I have one as a nursing student). What they are seeing is how inflamed the eardrum is, that will tell whether there is water there or not. Most likely you have Otitis Media, which simply means a middle ear infection.

I know what you mean when you describe the sensation of water. But rest assured that unless the eardrum is punctured the water will not go behind the eardrum with what you had experienced.

The antibiotics should be working in about 24 - 48 hours which will help relieve the inflammation of the eardrum.