I don’t think there exists any specific recommendations for ocean plastic pollution, and without a-priori arguments for why that cause area is potentially very promising, I don’t expect there will be a thorough one anytime soon.

In general, I think if you find yourself wanting to donate to a narrow cause area for which you don’t have strong evidence that it will be globally very effective, it is a good idea to ask yourself why you are actually interested in giving to that domain. Separating fuzzies from utilons is a phrase sometimes used around here, which suggests that if you have reasons other than effectivenes to donate to an area, that you should aim to actually optimize for that (such as the feeling of warmth of helping other people), and then separately optimize for doing the most good per dollar.

In this case, my suspicion is that you have other reasons for wanting to donate to an ocean plastic pollution charity, and that you would be better served introspecting on those reasons and seeing whether you can pursue them separately. Obviously, I can’t read your mind and I might be off here, so I apologize if I am misinterpreting you here.

If you're looking to decrease the amount of plastic entering the ocean, I'd suggest looking to improve waste management in Asia. The vast majority of plastic entering the ocean is from mismanaged waste in China. https://www.statista.com/chart/12211/the-countries-polluting-the-oceans-the-most/