I have been involved in beach cleaning and the vast majority of plastic found washed up was from the fishing industry, I would perhaps to go as far and say almost all plastic found washed up was from the fishing industry. Obviously that won’t be the case in developing countries where people throw their garbage into rivers.
As shown in the below video a lot of the garbage the found during the ocean cleanup operations was also from the fishing industry. You have these big fleets of fishing boats (mostly Chinese and Russia) that go around the world staying at sea for months or years at a time, they resupply and transfer their fish to a mother ship, these boats are not that big, as they don’t have much space I find it highly unlikely that they will be storing their garbage and transferring it to the mother ship, they just be throwing it over the side.
These days I think very little plastic in the water will be coming from cargo ships, commerical seafarers are well educated in proper garbage disposal and there is usually lots of space to store it or they have incinerators. But the great pacific garbage patch will keep growing as long as the fishing boats keep throwing their rubbish over the side, it is very hard to police and stop them doing it, they don’t have much space on their boats and they don’t have incinerators so practically they they couldn’t keep it onboard if they wanted to.
