Why do you hear the sound of the ocean in a seashell?

There are many theories as to why this happens. The best explanation for this phenomenon is that there is always just... noise... around you at any time. When you hold the seashell over your ear, you are physically capturing the noise which resonates inside the shell.

That is why you may here the wave-like sounds differently with shells of different size and shape.

Maybe this will spoil it, but... you really don't need a seashell to create this effect. the seashell to hear the noise. Use an empty cup or cup your own hand over your ear. You can even vary it by varying the distance of the cup or hand from your ear.