Because it can be effective growing medium. Now as I said I have no idea on ecoli or how they extract the oil.
But sea has water and food in abundance. Light also penetrates deep into water. Algea blooms for example can be many 10s of meters thick. You cam also use waves as a cheap and efficient way of mixing the water up and pulling nutrients up from the deep sea. Also the closer to the equator the more sun light you have, as well as we don't really use sea space for much.
It was merely a suggestion.