The health of the planet’s oceans is already threatened. Increased global temperatures and ocean acidity are both going to greatly impact many species of fish and many popular species are already threatened by overfishing.
Scientists estimate that 90% of stocks are already fished at or beyond sustainable limits according to information provided in the question. Decades from now when the average air temperature is even warmer and the ocean is even more acidic, ideally we need to have already developed algae and other alternative protein sources because replenishing the numbers of over fished species may not be possible in an ocean that is hotter than average and more acidic then it has ever typically been.
Reducing our cumulative human carbon footprint would help protect the ocean from temperatures increasing as high as worst case estimates predict, and help prevent the amount of acidity from reaching even more elevated levels.
Reducing the amount of acidic fertilizers used on farms and other landscapes would also help reduce the amount of acidic wastes washing off into the ocean or groundwater.
Reducing the use of hydraulic fracturing methods could help reduce toxins from leaching into the ground water and reduce the amount of methane being allowed to inadvertently escape into the atmosphere.
Regulations, regarding which species of fish and how many are allowed to be caught, are necessary to help support the rebound of the species that have already been depleted by over-fishing.
The bleaching of coral reefs is temperature related but acidity is also involved so modifying our use of acidic fertilizers might help reduce their impact on coastal regions of the ocean. Limiting activity in coral reef areas has helped some coral reefs to recover. Oil spills and heavy metals and other toxins and destructive fishing practices have all been found to be harmful to coral reefs.
The ocean is like our life blood and like our kidneys for detoxifying the planet’s air and water supplies, what is harmful to us can be harmful to it. 
Cleaning up the giant floating island of garbage in the ocean is a priority — humans caused that, we can’t deny that fact, so we should accept our responsibility for cleaning it up.
20 Science Questions for the 2016 Presidential Candidates --by Shawn Otto for sciencedebate.org; See the questions here: #18.
20 Science Questions
that U.S. presidential candidates were asked in 2016