Ships were allowed to dump into the sea but not within close range to territorial waters. But modern Cruise liners are run like cities. They have zero waste. All human waste and other waste gets recycled, cleaned. They have complete recycling plants & incinerators. If you go n a cruise, the Ship's Captain will give a presentation about 45 mins on how the ship is run. The plastics etc gets compacted and once the ship docks the recyclers wait to collect these.
The arrogant Chinese bastards bring shiploads of shit from the mainland and dump in the sea. Don't even try to compare the two or justify the act. It's totally wrong.
I do not think the US does this, or even the Russians. The Indians use the sea to dump all sorts of things. We aren't in the clear either. The largest ocean polluters in the world (mainly plastics) are China, Indonesia, Phillipines, Vietnam, SriLanka.....