Sailing a boat under a bridge - GIF and video

[Moby Duck]

They are possibly water bags. Secure bags to a mast top rope, dangle over the side, fill with a hose or bucket, (or just dip them), hoist bags slowly when full and they should give the required list if you got the calculations right. On completion, haul bags inboard and spill water on deck.
This is a well thought out maneuver but this yacht is not 'sailing' under the bridge, it is 'motoring'.

[Frank S]

Not as quick and agile as she thought she was

[ranald]

but she saw it in the movies.