How much is a league in metres/centimetres?

not sure, but read somewhere that it's about 1league is 6400m but somebody told that it's about 10km

111 120 000 meters, 11 112 000 000 centimeters.

Google. Use it.

A League is approximately 5 kilometres according to google, but the deepest water in the world is Mariana's Trench which is 11km deep.
20,000 leagues = 100,000km...
Even for a book, that seems pretty unrealistic having submarines travelling 100,000 km underwater... (or even just that amount of water existing!)

It's a very simple and effective process, really.