Around 1977 on the Flooded Mine ride, I recall the boats having triangular brackets on the front and rear, projections on some of the animatronics faces, and a part of the ride in the air shaft where boulders fell on the boats from the top of the shaft, but were diverted by a net just above. The boulders were apparently made of foam or light balls.
After watching a YouTube current pov of the ride, boulders no longer fall on the riders. Instead, it's populated by a scene of prisoners trying to climb up into the air shaft towards the end of the ride.
This is the same shaft that can be seen from the outside of the ride.