

I have no clue what the different Fairphone cameras are like but in general it doesn’t make sense to compare two different models by looking at the megapixels only. That says nothing about the image quality.
4K images just have ~8 megapixels, full HD only ~2 megapixels. More pixels just make the picture bigger, not necessarily sharper, better colours or more natural.
What does it dissolve into then? Plastics are full of carbon usually. If it leaves no trace (my interpretation: no solids, especially no microplastics) in the sea water, I would expect it to dissolve into some kind of gas. But gasified carbon very much sounds like carbon dioxide (or worse stuff like methane).
Furthermore, if it doesn’t need to be water-resistant, why use plastics at all and not paper or - if it’s meant to be transparent and foil-like use cellophan (cellulose hydrate)?