How can waterways be fuel efficient aand envirnment friendly when ships pollute the oceans by oil spills

Waterways are an important means of transporting goods within and outside the boundaries of the country. Infact sea ports handle over 80% of all cargo traffic in our country. It is an efficient and an environment friendly mode of tranport because there is no congestion on water ways unlike roadways. Since there is no congestion, there is no fuel wastage and pollution. 

Yes, at times, oil spills do pollute the sea water and affect marine life but such cases do not take place freequently.