Overfishing
Humanity has thrived upon consuming fish for a very, very long time and, in the process, has eliminated approximately 90% of the big fish population. Yes, we have reached the point where we have eaten entire species out of existence. Additionally, the ways by which we collected our prey destroyed entire ecosystems, as well. Trawling, which marine conservationists compare to bulldozing, grabs anything in its path - resulting in a hawl that is composed of only about 10% of the fish that was sought out.
Before solid and liquid waste can even be considered, the oceans absorb a third of humanity's CO2 emissions, drastically altering the pH balance of its waters. Then factor in the cigarette butts, netting, plastic soda rings, and all the other solid waste that gets thrown away or washed in from landfills - plus pesticides, nutrients, and other agricultural waste from farms - and you have a major problem. The water's chemistry is thrown off, creatures that survive the new acidity get caught up in the debris, and the waters continue to get dirtier and more lifeless.
If all of that wasn't bad enough, coastal communities insist on putting up brand-new beach-front resorts, condominiums, super markets, and goodness knows what else, all in the name of profit and tourism. Yes, tourism is important, but not so much as to continue without any regard to the environment, especially when a tiny rain storm can blow all of the construction waste right into our precious oceans. Restrictions need to be set on how close to the water we can build and what protective measures need to be taken.