In the beginning everything was simple. Bt maize was supposed to be just on the field, and nothing else would be affected: No organism that wasn't on the field, no organisms that would prey on maize pests. But over the years things became more complicated and now it's common knowledge that parts of the Bt plants make it off the field themselves, that predators can be affected indirectly in the food web. And still the question whether the field crop maize could also water organisms seems to be one step too far for most risk assessments.

After a study confirming the presence of Bt maize in headwaters in agricultural areas and adverse effects on caddies flies by Rosi-Marshal et al. (2007) and one about adverse effects of MON810 on Daphnia by Bøhn et al. (2008), there is now a study showing that mussels take up the Cry1Ab gene.
Douville et al. (2008) found that mussels Elliptio companata in an intensive maize growing area were significantly contaminated by Cry1A and Cry1Ab genes in their gills, digestive glands, and gonads. They found the transgene in surface water and sediment samples, and concluded that the exposure to the Cry1Ab gene seems to proceed by ingestion of microorganisms during feeding.
Douville M., Gagné F., Andre C. & Blaise C. (2008): Occurrence of the transgenic corn cry1Ab gene in freshwater mussels (Elliptio complanata. Ecotoxicology and Environmental Safety: doi:10.1016/j.ecoenv.2008.02.006.