There is a big difference between an inch or a foot in terms of how much trash is needed to elevate the entire planets ocean level. A large volume of trash in the ocean isn’t submerged, but floating on top of the water. Regardless I strongly doubt that there is enough trash on this planet that could be dumped and submerged into the ocean that could result in water elevation like you say. It sounds plausible at first but it really isn’t as plausible as you think it is. Don’t get me wrong, I absolutely agree that we need to stop littering the ocean. But for reasons other than water elevation.