Impossible.

On the way to Route 224, it's not a HM, but I captured a wild Dewgong Level 50 that knew Dive.

Not possible my friend, you can obtain them with an Action Replay or Gameshark device.

I'm sorry my friend, but it's impossible to obtain 'Oak's Letter' without cheats, the events were only in the US and Japan, but if you would like, I could send you a Shaymin from my Pokesav, trade me a Murkrow for it, I'm looking for a Murkrow, thanks.