the initial resolution is higher for SLC, that is true. But it is unlikely that information which is present in SLC is lost after conversion to GRD. What is basically done is converting the rectagular pixels to squared pixels.
Have a look at this post: GTC (Geocoded terrain corrected) data
If the plastic is the size of a single pixel you might get no result anyways because of effects of speckle and waves in SAR data. A mean filter which enhances bright pixels might be suitable but that depends on the smoothness of the water surface.