Complex sentence: The problem is that the water is also likely to be, at best, moderately contaminated with a small film of oil on the top of it or, at worst, substantially contaminated by a thick layer of oil.
Split into simpler sentences: The problem is that the water is also likely to be. At best, moderately contaminated with a small film of oil on the top of it or, at worst, substantially contaminated by a thick layer of oil.