Here is my video playthrough of this. If you're looking for more, such as how to get here, or what to do after, check out my walkthrough of the game!
1. This is after you hit the waterfall, and push the buoy out of the way.
2. Slowly move up towards the waterfall, and lower yourself.
3. What I found important was to always be moving forward in first gear. The engine will never overheat and you'll be slow and steady.
4. Get the boat just above the ocean floor and move on.
5. You'll come to a section that you'll need to lower yourself. I stopped my boat just before the wall, when it looked like the tail end of my boat was still going to hit the cliff behind me. It didn't.
6. Slowly lower yourself, then press on.
7. You'll then come to a slow incline. Again, take it super slow and steady.
The video should help. :)