Is there a train that goes underewater and takes you to Paris from London?

No, there is no underwater train that goes from London to Paris. The Channel Tunnel, which is a rail tunnel that connects Folkestone, Kent, in the United Kingdom, with Coquelles, Pas-de-Calais, near Calais, in northern France, is not an underwater train. While it does go under the English Channel, it is a train tunnel, and passengers travel on regular Eurostar trains through the tunnel.