dont throw garbage in the ocean recycle and make sure the products you use dont harm the oceanic ecosystems
A really good start would be the sincere application of the three R's: Reduce, Re-use, and Recycle.
Water treatment processes include: Filtration Settling Reverse osmosis Distillation De-ionization Softening Flocculation Chlorination Desalinisation Ozonation Activated Sludge Carbon treatment...
http://wiki.answers.com/Q/What_are_the_solutions_for_water_pollution"
people littering
Kill plants