A three-letter password is given at the start of each level. You can continue the game from any level you have a password to.
Password Screen
At the start of the game, go to the Password Screen (swim to the left), and enter the password. Select END and press to continue the game from the level for which you entered the password.
A three-letter password is given at the start of each level. You can continue the game from any level you have a password to. 
to continue the game from the level for which you entered the password.