The Polytechnic University of Marche, in Italy, researched the issue and learned that up to 6000 metric tons of sunscreen every year is settling onto the ocean beds. This wouldn't be such a problem, if it wasn't for the chemical reaction between the ingredients of sunscreen and the algae viruses on coral reefs. If this occurs then a coral reef could be completely killed within four days.
Damaged Coral Reef
Many resorts and companies around the Yucatán Peninsula have a policy of only allowing biodegradable sunscreen in their waters. Some operators will simply confiscate non-biodegradable products upon sight, whilst selling the biodegradable sort behind their counters. This is naturally more expensive and has led some tourists to scream, 'scam!' The more smugly selfish and ignorant even resort to lathering themselves in the destructive brands before they leave their hotels rooms, so that no-one can check whether it was biogradable or not. They then set off to potentially destroy the coral reefs, whilst feeling very proud that they saved themselves $9 USD.
The harmful chemicals are familiar to most popular sunscreen brands. They are PABA, octinoxate, oxybenzone, 4-methylbenzylidene camphor, oils, chemicals or the preservative butylparaben. All of these are damaging to the coral reefs. Even a small amount can react with the algae. This raises the odds to it being 15 times more likely that a virus will develop and, if that happens, then the coral will bleach and die. A coral reef risks contamination every single time someone swims in ordinary sunscreen. It's a timebomb waiting to go off.
Since 1968, The University of North Carolina has been conducting long term studies. Their conclusions are startling. 75% of the world's coral reefs are in the Indo-Pacific region, where they are dying off at a rate of 1% a year. That's nearly 600 square miles (1,553 square kilometers) of reef disappearing annually. In Australia, the Great Barrier Reef was covered in tethered plantoons (huge canopies) for two years, between 2004-6, to protect it. As the results were 'encouraging', the authorities are now considering plans for permanent covers. In the Caribbean though, where the local economy relies so heavily on tourism, the situation has gone critical. They daren't cover their coral reefs, when so many people come specifically to see it; while swimmers are contributing to higher concentrations of sunscreen in the water. The best that is currently being done are attempts to promote biodegradable sunscreen in the oceans, lagoons and cenotes.
Vibrant Coral Reef
Coral reefs are very pretty to look at. Most people who go snorkeling or diving are going to see the pretty reefs. However, there is much more to them than that. Coral reefs are often referred to as the rainforests of the ocean. Their biodiversity keeps everything ticking over nicely. Their presense controls the amount of carbon dioxide in the world's oceans. The oceans are currently the greatest sink of carbon dioxide that we have. The seas pull it from the air and store it there. However, in tropical waters, the opposite can be true. If the oceans become too saturated, then they start to release carbon dioxide into the air.
The coral helps to minimalise this. Without the coral reefs, you would be breathing in great lungfuls of carbon dioxide throughout the Yucatán Peninsula. The reefs also provide homes for many different species of marine life, which would simply become extinct without them. Many of these fish are caught to feed human beings and yes, they are covered in your sunscreen at the time too. Finally, reflect upon why coral reefs are often called 'barrier reefs'. It is because they stand between the bulk of the ocean and the coastlines; they slow the water down so that coastal towns aren't inundated.
In short, using ordinary sunscreen whilst swimming in the sea is the equivalent to leaving unattended open fires in the Californian forests or the grasslands of Western Canada. Yes, it might be ok. It might... Alternatively, you could just use biodegradeable sunscreen, which has none of the harmful chemicals, thus protecting both your sensitive skin and the coral reefs.
Biodegradable Sunscreen brands: BATAB, Cactus Juice, Caribbean Solutions, KissMyFace, MexiTan, Smartshield, Soleo Organics and UV Natural. They will probably be cheaper if you buy them on-line before you leave home, than if you have to make a desperate purchase in order to be allowed on a Cancún snorkeling trip.