What sargassum is and why it arrives
Sargassum is a brown seaweed that floats in open water and forms enormous mats across the tropical Atlantic, held at the surface by small air bladders. Since 2011 those mats have multiplied strikingly, and every spring the currents push them towards the Caribbean, where they strand on any coast facing east. Out at sea it is an entire ecosystem, with juvenile fish, crabs and turtles living inside it; the problem appears when it reaches the shore in quantity and begins to rot in the sun.
As it decomposes it releases hydrogen sulphide, which is precisely what produces the characteristic rotten-egg smell, and the water at the shoreline turns brown, cloudy and unpleasant to walk into. It is not toxic to swimmers under normal conditions, but the smell is strong, the beach becomes uncomfortable, and some people get skin irritation swimming through decomposing weed. Hotels remove what they can each morning with machinery and floating barriers, but none of them controls what arrives before dawn.
The calendar, month by month
The pattern repeats every year with variation in intensity, not in timing, and that is what makes planning possible. The first patches arrive in late April, the heavy season runs May to August, September and October usually ease off although they coincide with the real peak of hurricane season, and from November to March the shoreline is clean. December, January, February and March are, without any argument, the best months on the Mexican Caribbean coast for anyone coming for the sea.
Intensity does vary enormously from year to year, and there no forecast is possible. There have been near-clean seasons and seasons with record floating biomass in the Atlantic, and there is no serious way of knowing in December what the following May will bring. Which is why the only reasonable strategy is to choose the month rather than the hotel: in low season the price compensates for the risk you take on, and in high season you pay considerably more but buy a real certainty about the shore.
Which coasts escape it, and why
Geography explains almost everything worth knowing. Sargassum arrives pushed from the east by the current, so beaches facing open water in that direction invariably take the worst of it: the mainland strip from Playa del Carmen to Tulum, the coast at Mahahual, and much of the Riviera Maya corridor. Any shore sheltered by an island, a headland or simply by its own orientation receives a very small fraction of the same load.
That means enormous differences within the same region on the same day and in the same wind. The west coast of Cozumel, facing the channel, is usually far cleaner than the east coast of the same island, which looks out to open sea. The north of Isla Mujeres faces the gulf rather than the Caribbean. Isla Holbox sits on the side of the current that does not receive the weed. And two places are never affected at all, simply because they are not the sea: the cenotes and the Bacalar lagoon.
Where to swim when the shore is covered
A trip to Quintana Roo in sargassum season is not lost; it is badly framed if it was planned exclusively as a beach holiday. The peninsula has hundreds of freshwater cenotes within half an hour of the coast, a forty-kilometre lagoon at Bacalar, reef that starts diveable at two metres, and a long list of archaeological sites that do not depend on the sea at all. The budget freed up by low summer rates covers a good deal of all that.
It is also worth knowing that sargassum affects the shoreline and not the reef, which is an important distinction. A hundred metres out the water can be perfectly clear, and diving and snorkelling run normally provided you enter from a boat rather than walking in from the sand. Plenty of operators in the area work that way all summer without difficulty, and some of the best dives on the coast happen precisely in the months when the beach is at its worst.
How to check before you pay
There is no reliable long-range forecast, but there is very useful short-range information. Public satellite maps track the Atlantic weed mats with weekly updates, and local groups post daily beach-by-beach photographs on social media, with date and time. Three or four days out you can tell reasonably well whether a particular beach is clean or covered; three months out, absolutely nobody knows, however much they are paid.
What you can do with three months of margin is reduce the risk systematically: choose the month, choose a coast sheltered by geography, and choose somewhere to stay that does not need the shore in order to work. Asking the hotel directly is reasonable, but read the answer with judgement, because almost none will tell you their beach is covered. Dated photographs from a local group are worth considerably more than any commercial reply by email.
The mistakes people keep making
The central mistake is believing that price buys a clean shore. It buys nothing of the kind: the weed reaches a three-hundred-dollar-a-night hotel exactly as it reaches a sixty-dollar one, and the real difference lies in the machinery clearing it at six in the morning, not in whether it arrives. The second thing that always fails is reading the calendar, because plenty of people see May and June as low season for tourists rather than high season for sargassum, which is what they are.
The third mistake is about reaction, and it happens once you are there. People who arrive and find the beach covered tend to spend two days waiting for it to improve, watching the sea from the terrace. That is exactly the time it would have taken to reach Bacalar, see three cenotes or cross to Isla Mujeres. The peninsula offers water alternatives within three hours in any direction, and the decision to change plan has to be made on the first morning, not the last but one.
What people always ask
Six concrete questions about sargassum, answered without softening what cannot be fixed and with figures from recent seasons. They run from the most practical to the most uncomfortable, and the last is the question everybody asks at the end of the conversation: whether this is going to get worse over time or whether it is a passing phenomenon that will resolve itself in a few years.
One thing that does not fit in a short answer is worth adding. Sargassum has changed how people travel to this coast and is probably permanent, but it has changed nothing inland. The cenotes, the Maya cities, Mérida, Valladolid and the Bacalar lagoon are exactly as they were twenty years ago, and they are the better half of the peninsula. Planning the trip around them, with the beach as a bonus rather than the axis, is how you stop any given May from ruining it.