Due to Coral Bleaching
If coral dies due to bleaching, then that means that a wide range of marine creatures that rely on the corals exisctence (food, shelter) will have nothing. This could mean that these creatures have to either miagrate to somewhere that does have healthy coral or may simply result in mass extinction.
Mutual Dependence Breakdown
All animals and plants have temperature limits and when these are reached, natural processes may break down. On coral reefs, temperature changes can affect the relationship of mutual dependence between some animals and the algae which live within their tissues.
Base of the Food Chain
Other organisms effected by temperature change include krill, an extremely important link at the base of the food chain. Research has shown that krill reproduce in significantly smaller numbers when ocean temperatures rise. This can have a flow on affect by disrupting the life cycle of krill eaters, which in turn causes food shortages for higher predators.
Warmer sea temperatures are also associated with the spread of invasive species and marine diseases. The evolution of a stable marine habitat is dependent upon myriad factors, including water temperature. If an ecosystem becomes warmer, it can create an opportunity where outside species or bacteria can suddenly thrive where they were once excluded. This can lead to forced migrations and even species extinctions.