Using a little bleach on a boat ain't quite the same thing - it DOES evaporate and break down in sunlight very quickly.
You are rationalizing. One boater pouring bleach into the water may not be a disaster, but what if every boater poured bleach into the water? As a sailor, your attitude should be to be part of the solution, not part of the problem, IMHO.
Bleach is highly alkaline, with a pH of 12. Normal water pH is a neutral 6 to 8. Fish are extremely sensitive to changes in pH, so the pH level rise that comes from adding such an alkaline substance to the water can raise it to an unsustainable level. According to water experts Lenntech, high water pH chaps the skin of fish and a level of over 9.6 can cause death, inability to pass metabolic wastes and cause damage to gills and eyes. It can also harm juvenile fish development, as well as aquatic plants.
Chlorine bleach by itself is highly caustic and toxic to fish. It reacts with cell membranes and proteins, and then breaks them down. Since fish "breathe" water through their gills, the bleach enters their bodies and affects both their insides and outsides. In addition, it eats away the protective, slimy covering on the outside of most fish's bodies. This critical coat protects fish from disease and parasites. All of these internal and external actions of bleach eventually kill the fish. The material data sheets for chlorine bleach from manufacturers such as Corcraft and Sentry Industries warns against releasing its product into surface water due to its toxicity to aquatic life.
Chlorine bleach breaks down and forms organochlorines in water. One particularly toxic organochlorine is dioxin. These compounds are toxic to all life, including aquatic life. The fish ingest these compounds and become contaminated, which then contaminates any creature that eats them. Organochlorines have been linked to cancer and developmental disorders, as well as neurological, immune system and reproductive issues.
Effects of Chlorine Bleach on Aquatic Life | eHow.com