Indeed.. prevention is better than cure. That should probably a principle for engineers. But anyway, we should still think of a way to fix the existing pollution problem.
Regarding chemicals.. maybe we should approach it like how oil spills are cleaned up. One can use "sorbents" to absorb the chemicals from the water like a sponge. Another method is to use biological agents to break down the chemicals into harmless compounds.
Of course, thats as much as I can contribute regarding to technical stuff
I'd have to research these methods more thoroughly to see their feasibility in cleaning chemicals (or sewage) from the sea.