"meant to survive rain storms" doesn't really mean the same as "meant to survive flooding", especially not "meant to survive flooding with salt water". For example, a car survives outdoor weather year-round, but you probably wouldn't want to buy one that was submerged in a flood zone. This plant seems to be floating, though, so they stay above water regardless.

Do you have any specifics about the implementation that makes you say that the engineers have not considered this?