In real life, ocean life and flora actually removes more carbon from air (idk), but in factorio it seems to remove no pollution at all and that is annoying as hell!!
This little mod will boost water pollution absorbing properties
(chunk of deep water will now absorb roughly same amount of pollution as chunk of dense forest [8p/m])

This mod was hastily ported to 2.0! although absorption is already boosted almost 5 times from 0.3p/m in 1.1 to 1.5p/m in 2.0, maybe you still need more.
something got changed about how tiles are prototyped and now all water tiles inherit change of a single tile, witch is cool ig but i couldn't find relevant documentation