Where do I find rock smash in Pokemon Snakewood?

Solution 1:

Rock Smash is HM06, which is in the guide as kindly posted by Robotnik, where it states:

Upon defeating the leader of the base, Maifenense, the player returns the stolen Pokegear to Gleis, who gives him HM06 and an escape to Mauville City.

So you'll need to find Gleis to get Rock Smash. You'll find him after defeating all the Deadly Seven members in a repetitive puzzle, as shown in this video, which you'll have to follow from Part 20 to 22 in order to find Gleis. Good luck!

Solution 2:

Go to the lake by the walking road, directly south of Mauville. Start SURFing, and there will be an island with a sign on it. Walk into the indent in the hedges and it will allow you to enter the "Island of Calm".