People leave all sorts of junk behind on coastlines and in the water, and it causes problems. The California Coastal Commission warns that debris in the water can wrap around boat propellers, causing engine damage, and that trash like cigarette filters and grocery bags look like food to animals. Once ingested, they cause suffocation or starvation. The point is, be fastidious about bagging all your detritus and bringing it home with you -- for recycling and composting, if possible. If you're really committed to protecting the environment, pick up somebody else's trash as well.