Three main causes of preventable ocean deaths

Plastic

Plastic is very dangerous because  a sea animal may think it's food and eat it .  It therefore can easily block their airway/gills causing it to drown .  Putting that ONE plastic bag in a BIN can save a life!

Fuel pollution

Any boat, anywhere can and will pollute the ocean! The captain may have no intention of doing that but he/she can't stop the boat doing what it needs to do, to work! The fuel ejected from the engine creates a horrible environment for fish to live in, therefore they die! 

Fishing nets

Some fishing nets are stuck to the ocean floor and dolphins, seals and other sea mammals can get stuck in them! If they cannot get up for air they will die!