Why does water go up your nose but not into your ears while swimming?

The ears are indeed connected to the back of your throat by a very narrow tube called the Eustachian tube. It goes from the area just behind your nose, so where the nose joins the back of your mouth, out to your ears. But it goes to the bit of your ear on the inside of your ear drum, not on the outside. The outside of your ear canal, the bit where you can stick your finger in and wiggle it up and down, that ends in your ear drum. This forms a solid layer which separates your external auditory canal, which is the bit you put your finger in, from the inside of your ear. That means that there is no way for water to get in there at all. If you do get water in your ears, it's because some water has got up your nose and trickled down this very fine Eustachian tube and got into your ear. You might then want to know why your ears go pop when you go up in an aeroplane. The reason for that is because the Eustachian tube is quite narrow, the pressure pushing on the outside of your ear compresses the air on the inside of your ear by squashing the ear drum. The idea of the Eustachian tube is to let it equilibrate. If you have a cold or a nasty infection in the sinuses, it can block off the Eustachian tube with some mucus. That mucus can stop the air pressure equilibrating, and that's why you have that funny popping sensation. You can equalise it if you hold your nose and blow hard against it. This forces air into your ear and helps to equalise the pressure. But there is no connection between the outside world across your ear drum.