Stop gates are an alternative to stop planks. Permanent gates each side of aquaducts and embankments, designed to swing, or be swung closed in the event of a breach A set at the entrance could seal off the marina more quickly. Most on the system are in an advanced state of decay, which suggests they may not work well, or CaRT think the probability of a breach is so low that the water they would save doesn't justify the expense. The marina would have to budget for a new set every 25 years, or so, at a cost similar to a pair of top lock gates, plus maintenance clearing silt from the cills, so they could stand a chance of working. Good luck with making the case to them.
