Plastic bags that are submerged but floating in the water resemble jellyfish. Immature turtles and whales have been found to mistake them for jellyfish, and this can cause death by starvation.
Plastic bottles are often mistaken for cuttlefish given their tough but chewy texture. Again, it's whales, maybe turtles too who make this often fatal mistake.
Plastic Bits. Birds like the albatross often confuse plastic bits for food too! The animals fill their bellies with plastic of some sort and have a false sense of having their hunger satiated. They die of malnutrition.
Entanglement is another large issue. Rope from ships, fishing lines and broken nets are the culprits, entangling turtles, birds, snakes, crabs and fish.
Habitat impact. Just like you would not want to live in a home filled by others' inconsiderate litter, a natural environment means a safer and healthier home for plants and animals.
Scenery. The beach is not pleasant if you are swimming in the sea and have a plastic bag wrapped around your legs. And walking down a beach littered with trash robs it of the tranquility and beauty the sea and waves otherwise offer.