You're taking the ph of pure water (pH = 7) as your boundary between "acid" and "basic". That is an arbitrary choice and depends on a medium (pure water) that is not found outside the laboratory. Sea-living organisms in pH = 7 would esperience it as an acidic environment, since they are adapted to a pH of 8.1 (by your claim).

There's no answer I could possibly provide you here. Where are you going for with this? The pH of 7 as watershed is a chosen standard, and really only somewhat arbitrary because it is the natural state of pure water by its dissociation behaviour.

The descriptor that sea-living organism would experience a pH of 7 as acidic, is simply the incorrect use of the definitions. They would experience it as less alkaline, or neutral. Would the pH drop below 7, then they would experience an acidic environment. It's just language, semantics. When I read that article, I read a language that simply doesn't make sense to me, knowing that ocean pH hovers at about 8.1. It's actually a futile point anyway, because what really matters is the change in the equilibrium reaction between carbonate (CO3), CO2 and bicarbonate (HCO3) which directly affect the dissolution of calcoimcarbonate shells.

I've no qualms to describe the process as acidification, and I've even done so in my previous post by stating that the Royal Society's use of acidification is perfectly correct. Because that's what it is: the lowering of pH in water.