Larger Organisms
On many fur-bearing animals (like seals) the oil destroys their insulating abilities (which depend on their fur being clean) and therefore kill many of them off through diseases like hypothermia. Seabirds are also largely affected in a similar way to the mammals as the oil destroys the water-repellency that these birds possess and also often leads to hypothermia. Fish also are another main culprit of these disasters with the oil leading to things like reduced growth, enlarged livers, changes in heart/respiration rates, fin erosion, reproduction impairment, as well as late egg development and low larval survival.
Corals and Mangroves
Corals and Mangroves are both highly damaged by these spills. The chemicals in the oil damage the organisms, which are both very slow to regenerate, as well as releases trapped CO2 through the death of these species. Not only this, many organisms rely on these species as a form of habitat and have symbiotic relationships with the organisms. This means that by destroying the habitat and food of other species the entire ecosystem is affected by this damage and could potentially never be restored to its former glory.
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By threatening so much of the environment and the organisms within it, these spills can have huge effects on the marine biome. If we are not careful and do not take these spills seriously they could (and already have) cause permanent damage to the wonderful underwater ecosystem and could have detrimental effects on the sustainment of our world as we know it.