Essentially the notion you just put forth was what the group more or less came to in last night's class.
The theory being that Gandalf, true to his ability to inspire or "enhance," augmented the vigor of the Loudwater's flood thus dislodging boulders. And he was worried he had done too much because of said boulders being dislodged.
There may well be spiritual water boulders along with the spiritual water horses. Though, I now like to think (thanks to the group) that he put his aesthetic efforts into the horses and his "augmenting abilities" into the vigor or the water and any boulders that are there are seen by all (meaning it is likely that Aragorn and the other Hobbits did not see Gandalf's horses).
The class may have lingered on boulders last night but I don't think it's about the boulders so much as an opportunity to analyze Gandalf's power when it is in play and accounted for. Matt DeForrest's direction to go back to the passage of Frodo's live account was smart.