There is a top secret area near the beginning of TTM that allows you to warp to where the first monkey is. When you start the level, run to the left, away from the path that takes you to the mushrooms. Press C-Left once, to shift the camera to the left. You'll need this to line up with the secret area. On the mountainside, you'll see a corner jutting out towards you. It's where the flat mountainside meets the dark, shaded portion that you can crawl on. Run down the beach, and just before you touch the water, long jump towards this corner. If you hit it directly, at just the right angle, Mario will disappear through the wall.

Swim forward a little (carefully), and swivel Lakitu around so you can see Mario. You'll see Mario swimming in a small pool inside the mountain. Swim around until you see the corner of the pool, which is diagonally to the right of where you entered. Now, swim over to it. You'll want to face parallel to the stream coming off of the waterfall outside. When you've positioned yourself correctly, swim off that corner. If you did it right, Mario should fly all the way up to the pool where the monkey is. He'll be in the wall, so just swim out, and jump onto the land.