Shellfish Aquaculture Improvement Act of 2021 This bill limits the ability of an aquaculture worker to bring a civil action against an employer for employment-related injuries. Specifically, the bill limits recovery for injuries sustained by an aquaculture worker if (1) state workers' compensation is available to such individual; and (2) at the time of injury, the individual was engaged in aquaculture in a place where the individual had lawful access. An aquaculture worker is an individual employed by commercial enterprises involved in the controlled cultivation and harvest of aquatic plants and animals, including the cleaning, processing, or canning of fish and fish products, the cultivation and harvesting of shellfish, and the controlled growing and harvesting of other aquatic species.