Rising sea level will kill coral reefs, because corals can't grow fast enough to stay near the surface.

Too much CO₂ in the water will inevitably kill corals. The temperature can also kill them. So it seems that coral has little chance of surviving to the end of this century.

A billion people depend on marine resources for their food. In a few decades it could be a billion and a half. Thus, the loss of most of those resources could easily kill a billion people.

They won't all die directly from hunger. Many of them will die in wars over control of the dwindling marine resources. These wars will be futile, in that victory will at most delay the death of the victors.

If they start sooner, and fight fossil fuel extraction and use, they might avert much of the disaster. However, if they don't limit their populations, then barring some unforeseeable event they will reach the starvation point sooner or later.