It looks as if the bags are closed tightly, how can they open by themselves in the pool?
The place where Francis emerges on the roof is not backstage, but right above Palm Beach's main entrance. He is climbing out of a wooden box they placed there.
Charles' plan to hide the money in the dressing-cabin was not worked out very well. It was predictable that the police would search every possible hiding-place around the casino. Handing over the bags at the swimming-pool would also be very suspect. Finally, what's the point hiding the bags into the pool? They would be visible from the surface anyway and would have been discovered immediately.
When Charles is entering the vault, M. Grimp is standing with his face against the wall, so he can't see neither Charles nor the bags. But he gives a very detailed description to the police later.