Other sub types?

[fvd]

Hi,

Is there a way to change to another type of boat?
Now i'm using the Narwhal but i would like to change to a Gato or Balao.
I know that you sometimes get a message that says you can get another boat, but is this the only way to change?
And when exactly get's the Gato available?

Thanks and regards,
fvd