It's not just heat, it's ocean acidification. As the concentration of CO2 in the atmosphere increases, CO2 in the water increases as well. When CO2 dissolves into water, it forms carbonic acid. This is the same stuff that makes fizzy drinks like soda taste sour.
So more CO2 in air = more CO2 in the water = lower PH in the ocean. I don't know the details but the more acidic water messes with the chemical processes that corals and other aquatic life depend on.
So more CO2 in air = more CO2 in the water = lower PH in the ocean. I don't know the details but the more acidic water messes with the chemical processes that corals and other aquatic life depend on.