The only reason I believe that transporting waste by ship is the best way to move it is because it is less of a drag on the environment in and of itself. What I mean is that garbage is already a pollutant, while ships really are not--compared to cars and trucks, at least.
If you absolutely, positively, have to move a stupendous, mind bending amount of material- ship transport is and always has been ones only real option. If the load must travel further than rails can take it, it must go by ship. It's not like a wreck could be any more toxic than the oil.
Yes, transporting waste by ship is the best way to move it, because ships can carry a large amount of cargo. It is amazing how much cargo can travel by ship, rather than by air or any other way. Ship is the fastest way to move the waste, especially if the waste is to be deposited off in the ocean offshore.
Transporting waste by ship is the best way to move it. They can haul a lot of it at once and they can pretty much take it to any destination that they want to. I think this is the best mode of transportation for it and it creates the best movement of large amounts.
There is no good way to transport waste, but I honestly don't see a reason for a lot of waste to be transported by ship. Waste is created anywhere humans live and while many people live near water, there are many that are not near areas where ships can access waste. I do not think this is the best way to move waste, mainly because it's not an option for everyone.