When I designed our 90/90 the sink was located directly above the sink sea cock, rubber hose between.
With the sink (10 inch deep) piping above the WL , any clog was simply removing hose from under sink and jamming a broom handle down to clear grease.
In 22+ years of living aboard , only can remember the need once.
My guess is any accumulated grease gets eaten by always feeding sea creatures.
An above the WL discharge would stay visible , for much of winter on top of the ice. UGH!