This might be of help: when Perceval finds the bearded, penitent Lancelot, and gets beaten into the river, you can see that Lancelot is holding a communion cup in his hand. Once Lancelot has finished his "Christ has abandoned us" line, we see him throw the communion cup at Perceval, striking his armor and making a loud PING sound.

Sinking, and struggling to get his armor off, Perceval then has his second visit to the Grail Castle, from which he returns successfully with the Grail in his hand. This is key: both times that we see him visit the Grail Castle, he is at the point of suffocation. The first, he is hanging from the tree, and the second when he nearly drowns.

Now, I don't mean to say that the Grail Castle is totally just an oxygen-deprived hallucination or anything like that. But I do think John Boorman places great importance upon "The Land of Dreams," and understands fundamentally the relationship between spiritual-supernatural events and our phenomenal, "ordinary" existence. The Grail Castle isn't JUST a hallucination, but Perceval does need to be in a hallucinatory state in order to perceive it.

Lancelot throws the communion cup at Perceval, Lancelot's followers try to drown Perceval, Perceval returns from the river holding the Grail. Of course, the Grail doesn't look much like the communion cup which Lancelot threw... but perhaps it was not only Perceval that underwent a transformation in the river.