I think a lot of the plastic in the oceans gets there from India, and possibly other states. I've heard China are guilty too. I watched a news item about India, and how in many areas they have no rubbish collections, so people throw their rubbish into the rivers, the Ganges is very badly polluted because of this, and of course the rubbish is taken away at high tide and washed into the sea. The bit of film I saw showed all sorts of plastics, plastic bags and other things. Last year I was in Cambodia and I learnt from our tour guide that there are no rubbish collections there either, the rubbish was all piled up along the road. The biggest problem is that although we should, and I hope do, everything in our power to reduce pollution of the seas, and in fact all pollution, we have to recognise that many countries in the world cannot afford, or choose not to afford, any kind of rubbish collection, let alone recycling!
With regard to cruise ships, I have cruised on and seen with my own eyes, extremely good and efficient recycling, and rubbish collection. They do not throw rubbish into the sea, maybe they used to, I don't know, but they don't now.