What is a rubber sheet?

In the context of a regular rectangular rubber sheet not otherwise worked or shaped then it is 40082190.
Is that still the case when it is part of a project that includes steelwork and controls that, when combined, produce a finished article that is ostensibly an inflatable system used to control river water levels?

If it is not integrated into another product, it generally does not change its heading.

Only if it belongs to a bigger product, and all parts are declared together (e.g. a kit, just not assembled yet), the whole bigger product can be declared as if it was a finished good.