Although there is no general consensus amongst scientists, most sunscreens are suspected to harm coral reefs and other marine life.
Harmful are not only the particles and/or chemicals that come off while swimming but also what travels through sewage systems when washed off in the shower. Also, as sunscreen chemicals are absorbed by our skin and excreted by urine, they end up in the ocean eventually through this route.
Unfortunately there is no scientific consensus yet on this issue, and what is the best alternative. Protecting yourself from the sun is important, especially in the tropics. If you want to protect marine life, the choice for good protective clothing and staying less in the sun is a far better idea than using a lot of sunscreen. Especially if you go swimming, diving, surfing, needless to say that good sun-protective clothing is better for you and for the environment.
For more info see sunscreen .