Animals can eat trash and die, or they can get entangled in it and become injured.
People don't like to see litter floating in lakes or in the ocean. It's no fun to go on a vacation and walk along a beach littered with trash. People can cut their feet on broken glass bottles. Plastic debris can get tangled in boat propellers, damage the engine and threaten the safety of boaters. A boat in the water that will not work can be very dangerous.