Many kids (hopefully) will be familiar with the Great Pacific Garbage Patch, but whether or not they have prior knowledge this is a good starting point. It's the story of a research voyage from the perspective of three student scientists. Miriam Goldstein, Chelsea Rochman, and Darcy Taniguchi are each researching a different part of ocean life as it might be affected by the garbage patch.
The book takes us through the research of each student, and also through daily life on board the research ship and how different types of ocean research are carried out. It also talks about possible impacts of plastic in the ocean and on earth and gives suggestions on how kids can help. Now, that last part I'm kind of meh about - it's the usual "use reusable bags" type thing which A. might or might not actually be helpful and B. most kids can't control those types of things, but it's a pretty common list of "ways kids can help the environment" and I get that you can't really explain the nuances to kids. Anyways.
Verdict: If you have kids interested in ocean science or science in general, this is a great book to introduce the general topic as well as the specific research ideas. It would work well for the entire range of middle grade, and the only drawback is that Milbrook's library bound titles are very expensive.