3000 containers/year is a pretty small drop in the ocean. Most dangerous chemicals that you don't want in the ocean are loaded in a way that they are surrounded by other containers, so they are less likely to fall off (and less likely to be shot by pirates and catch fire). It seems reasonable to assume that the vast majority of the sinking containers are filled with pretty harmless stuff
I saw somewhere that for loading the outer/upper containers are lighter ones, and generally consumer grade plastic crap (toys, shoes, etc). same video shows loads of this stuff arriving on the shores of Oregon, USA - just a little waterlogged.
My analysis of that analogy is that "ocean," being on both sides of the equation, simply cancels out, so basically it is saying that a 3000 shipping containers equals one drop. Which I'm not buying. :)