Lakitu generally takes you to wherever the nearest land is from the water you fall in. Keep practicing and I'm sure you'll pick it up eventually. Failing this though, and beginning back on the guardrail, do this process again but start with the spin turn to the right. Lakitu will tow you out of the Lake and dependant upon where in the Lake you landed, will take you to a later part of the course.
Jump with the R button before making contact with the guardrail, If done correctly you'll not only pass through the wall, but you'll land in a lake, perhaps it's vanilla lake, who knows. Now you will be able to view your driver as you would on the player selection screen from the side. It's at this point that Lakitu will turn up and tow you back onto the course, as he puts you down hold the A and B buttons down together and tilt the analogue stick to the right, your driver will then do a spinout, continue this spin until your kart faces ninety degrees to the right, then release both the analogue stick and the B button. Turn ninety degrees to the right and drive straight at the guardrail, but keep hopping until your kart lands atop of the guardrail. Using any driver select the map on your screen, and continue along the course until you reach the point at which the map layers (overlaps) itself. It's also quicker to go through this tunnel even if you don't have the star, unless your murdered by the train that is.
Just before entering the tunnel, activate your star power up, at one point inside the tunnel the lap counter adds a lap. If on lap one or two you get a star power up (If your in first or second its unlikely.) turn left on the second crossing, and head down the train tunnel. There is a small path of sand through the water on the right a second or two after passing the archway. When you arrive at the ramp which is just before the tunnel entrance jump just before you get to the end of it, although a boost is good to have it isn't a necessity. This is a very difficult trick to pull off, and will set apart the master karters from the good karters. If done right you'll jump the wall and cut the final curve of this course and save a fair bit of time in process. The sixth brick on your left is the one to look for, boost in it's direction then jump with the R button right before you hit the wall. When you emerge from the tunnel on Luigi Raceway, go towards the position where the grey wall meets the brick wall on your left. Luigi Raceway - How to jump over the wall
Mario Kart 64 (Nintendo 64) - Shortcuts & Glitches