Zinc concentrations are much higher in highly polluted industrialized coastal areas, like the North Sea (up to 70µg/l), than in the open ocean (0,1µg/l). Zinc usually enters the marine environment through effluents from smelters and mining sites.
Zinc tends to be bioaccumulated by bivalves. Especially oysters contain large amounts of zinc, in the highly contaminated English Restronguet Creek they contain zinc concentrations above 10.000 µg/g dry weight. However, like most heavy metals zinc doesn't tend to biomagnify, so that it causes little harm to sea birds or marine mammals. 
Case studiesThe relation between pollutants and disease in guillemots