After searching the net for info on the secret level, and finding
mostly questions, I figured someone might be interested in this...

This pertains to Marble Madness for the Apple //e.
The key to the solving the water-maze lies in two-player mode.

Getting to the water-maze is well documented elsewhere.  On level 1,
jump both marbles to the left side of the screen and wait in the right
place at the screen's edge.  The message will trigger at 13 seconds.

The water-maze...

To cross the river, touch the second 'island' from the left.  I found
it easiest to back up and charge into the river at full speed.  You
must adjust your angle of approach to compensate for the river's
current.  Floating on the boat/leaf is another method.  You will be
teleported to the water channel on the lower left.  Get both marbles
across.  Be sure the first marble allows room for the second's
arrival.

Next, maneuver both marbles through the water channel onto the top
ledge at the elevator.  Take the elevator down to the bottom ledge on
the right.  This ledge is four blocks wide with an adjacent red/blue
pillar.  Get both marbles to this ledge.

This is where both marbles are required.  Move the first marble onto
the pillar.  Then, when the second marble touches the rightmost white
square on the ledge, it will trigger the pillar to descend.

At the bottom, exit onto the blue platform and force both black
marbles off.  If successful, you'll get the message:


CONGRATULATIONS!
Huge bonuses being added...
The password in my next game is GUS


Your score will be 120,000+ and the survivor(s) will advance to level
2.

This level is extremely difficult and unforgiving.  I made extensive
use of ApplePC's save state function to complete it.  The elevator and
pillar descents are tricky.  You must adjust your position while
descending to avoid falling.  This appears to be a cooperative level
with the opportunity to be cutthroat at the end.

There may be variations to this solution...