I need to know how I can get a Pokemon that knows dig so I can do the underwater..

Diglet can learn dig without a TM,so you should catch a Diglet so you don't need to teach it dig with a TM.

Hope I help

You can get a Pokemon that knows dig in the desert. Trapinch.
It knows dig. And I think you can get a TM in Fallarbor Town. I said I think.
I am not sure. Ok your at the poke center. Go left. Go past the move tutors house.
On to the next route. Route 114. You come to a house just past the move tutors house. Go in.
Talk to the little kid and he will give you TM dig. Ok I do remember.
I know this 'cause thats how I solved the puzzle. P.S. After you get in there you have to have a Relicanth and a Wailord.
You put Relicanth in the first in you poke party. And put Wailord last in the party.
Go up to the wall and read it or something. (been awhile scince I last did this)
After this is done three caves open. One in the desert. To get that Pokemon stand in the middle of the braile wall. Take 2 steps to the right, 2 steps down use STRENGTH.One past Petalburg city.
(remember the path you took to go to Dewford town with the sailor? In that path)
To get this one you have to read the thing on the wall. It says something about waiting for time to pass or something. After you read that DONT MOVE!
Wait about two minutes. And it will open.
And the last one is on route 120. Go to the doc is for Mt. Pyre. Go left till you can't go left anymore. Go up steps. Keep going up. Till you see the cave. To get the Pokemon go to the middle of the room and use FLY. Weird huh?
Whew that was long..... Hope I helped!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!