What happens when the earth's oceans warm?
There could be many implications but I would discuss only two important points: 1. warm water would be able to hold less amount of oxygen, 2. there will be coral bleaching.
Oxygen escapes easily from warm salty water. Hence there will be decrease in amount of dissolved oxygen in water: as a result all aquatic gill breathing organisms would suffer. Population of gill breathing organisms would drastically decline, disrupting food chains.
Most productive oceanic ecosystems are made of corals, which harbour photosynthetic algae within their cells and appear colourful. In warmer water, intracellular algae are lost from corals making them dull and white, a phenomenon known as coral bleaching. Innumerable number of aquatic organisms live in and around coral colonies where they match the corals in colour and shape. Such organisms would immediately lose advantage they draw from camouflage and would perish eventually.
So warmer ocean means loss of biodiversity.