How Murky Does the Sea Get After Heavy Rain?
How Murky Does the Sea Get After Heavy Rain? Diving Koh Tao and Phi Phi During Monsoon Season Heavy rain is in the forecast. Does that mean you should immediately reschedule your diving trip?
Not necessarily.
Rain on land and underwater visibility are not directly connected. Some nights bring heavy rain, yet the water at certain dive sites can still be relatively clear the next morning. On other days, the sky may be completely sunny while underwater visibility is poor because of waves, currents, suspended sediment or plankton.
If you are planning diving in Koh Tao during the monsoon season or diving in Phi Phi during the rainy season, the question should not simply be: “Will it rain today?”
You need to look at the overall sea conditions as well.
What Makes Seawater Murky?
There are three main factors to consider.
1. Runoff and Sediment After Rain
Heavy or prolonged rain can wash soil, sand and sediment from the island or mainland into the sea. The effect is usually most noticeable close to shore, in shallow bays and near areas where rainwater drains into the sea. However, rain does not make every dive site murky at the same time. Sites farther offshore or exposed to different currents may be affected much less.
2. Waves Stirring Up Sand and Sediment
In many cases, waves have a greater effect on visibility than the rain itself. When strong winds continue for several days, waves can stir sand and fine sediment from the seabed, especially in shallow areas. This is why you can have a beautiful sunny day but still find poor visibility underwater. The opposite can also happen. It may rain overnight, but if the sea remains calm, some dive sites can still be relatively clear the following morning.
3. Plankton
Green or cloudy water is not always caused by soil or sand. Large amounts of plankton can also reduce underwater visibility even when there has been no rain that day. Plankton is an important food source for many marine animals, including some larger species. But it is important not to assume that: “Green water means whale sharks will appear.” Whale shark sightings depend on many factors and can never be guaranteed simply from the colour of the water.
It Rained Last Night. Can I Still Dive in the Morning?
In many cases, yes.
Instead of looking only at rainfall, check:
wind conditions
wave conditions
whether heavy rain has continued for several days
how close the dive site is to shore
the latest conditions reported by the local dive centre
Before cancelling because of a weather forecast, it is often better to ask: “What are the conditions like at today’s dive sites?” A dive centre that goes out every day usually has more useful information than the rain icon on your phone.
What Is Diving in Koh Tao Like During Monsoon Season?
Koh Tao is in the Gulf of Thailand, which means its seasons are different from those on the Andaman coast. The middle of the year is generally an attractive period for diving in Koh Tao, while sea conditions tend to become more unpredictable toward the end of the year, especially around October to December. That does not mean diving completely stops during the monsoon. Koh Tao has many different types of dive sites, including shallow reefs, deeper pinnacles and wrecks. This gives dive operators some flexibility to choose locations based on wind direction and sea conditions. Travellers staying for several days also have an advantage. If visibility is poor on one day, conditions may improve on the next. Someone visiting for only one day of diving has much less flexibility.
What About Diving in Phi Phi During the Rainy Season?
Phi Phi is on the Andaman coast, so its seasonal pattern is almost the opposite of Koh Tao. In general, November to April is the easier period for travelling around the Andaman Sea, while approximately May to October brings stronger monsoon influence. However, the rainy season does not mean you cannot dive in Phi Phi. Many dive centres continue operating, but the sites available on a particular day may change depending on wind, waves and sea conditions. On days when offshore sites are not suitable, sites closer to the islands may still be possible. So even if: “It is sunny in Phi Phi today,” that does not automatically mean every dive site will have clear water or every boat trip will operate.
Koh Tao or Phi Phi: Which Season Is Better?
A simple way to remember it is:
November to April The Andaman coast generally has the advantage. This includes areas such as Phi Phi, Phuket, Koh Lanta and Koh Lipe.
May to September The lower Gulf of Thailand is often the more attractive option. Especially Koh Tao, Koh Samui and Koh Phangan. This does not mean there will be no rain during these periods. It simply means sea conditions are generally more favourable on one coast than the other.
If the main purpose of your trip is diving or snorkelling, you may not need to change your travel month. Sometimes, changing coasts is enough.
What If Your Diving Trip Is Cancelled?
First, find out exactly what has been cancelled. If the dive trip is cancelled but passenger ferries are still operating, you may already be on the island and the dive centre may offer another site, a different date, credit or a refund according to its own policy. If the passenger ferry to the island is cancelled, the situation is different because you may not reach the island in time for your course. Contact the dive centre as soon as you know. The dive centre’s cancellation policy and the ferry operator’s policy are separate, so a cancelled ferry does not automatically mean your diving course will be refunded or rescheduled. Always check the conditions before paying a deposit.
How Should You Plan Ferries Around a Diving Course?
One of the most common mistakes is planning everything too tightly.
For example:
Morning ferry to the island → afternoon diving course
It looks efficient, but if the ferry is delayed by even one departure, the entire day can be affected.
If diving is the main reason for your trip, leave some time between arriving on the island and starting your course.
Another important rule is: Do not dive and then fly immediately afterwards. As a general guideline, allow at least 12 hours before flying after a single dive and at least 18 hours after multiple dives or several days of diving. If possible, keeping the final day of your trip free for resting, sightseeing or returning to the mainland makes planning much easier.
Check Ferry Routes Before Booking Your Diving Course
Koh Tao is connected by boat with several areas, including: Chumphon, Surat Thani, Koh Samui and Koh Phangan
Koh Phi Phi is mainly connected with: Phuket and Krabi as well as other seasonal island routes. On the Andaman side in particular, some ferry schedules change between the high season and monsoon season. Before confirming your diving dates, check the actual ferry schedule for your travel day. You can search and compare ferry routes with Gowego before planning the rest of the trip.
Is Snorkelling Still Worth It During Monsoon Season?
Snorkelling is particularly sensitive to waves and murky water because you remain close to the surface. If waves stir up sand or sediment in shallow water, visibility around reefs can quickly decrease. So if the main purpose of your trip is: snorkelling, seeing coral and finding clear water choosing the right coast for the season is more important than simply hoping it will not rain on the day.
Frequently Asked Questions
It Rained Overnight. Can I Still Dive in the Morning?
Often, yes. Rain during the night does not automatically mean the morning dive will be cancelled or visibility will be poor. Wind, waves and the conditions at the actual dive site matter just as much.
What Is the Best Time to Dive in Koh Tao?
The middle of the year is generally a good period, particularly around July to September, while conditions tend to become more variable toward the end of the year.
What Is the Best Time to Dive in Phi Phi?
In general, November to April is the easier period for travelling and diving around Phi Phi and the Andaman Sea. Diving is still possible during the middle of the year, but you should expect greater variation in waves and weather.
Is Diving in Murky Water Dangerous?
Poor visibility does not automatically make a dive dangerous, but it can make it more difficult to see your buddy, navigate and communicate underwater. If you are a new diver or are uncomfortable with low visibility, tell your instructor or dive guide before entering the water.
Final Takeaway: Don’t Look Only at the Rain Forecast
If you are planning diving in Koh Tao or Phi Phi during monsoon season, do not make your decision based on rainfall alone.
Underwater visibility is also affected by: wind, waves, currents, sediment, plankton and the location of the dive site Some rainy days still have good underwater visibility. Some sunny days do not. If diving is the main reason for your island trip, the most useful things you can do are: choose the right coast for the season, leave some flexibility in your itinerary, check ferry schedules and ask the local dive centre about the latest sea conditions. Because when it comes to the ocean, rain is only one part of the story.
Getting to Koh Tao
Getting to Koh Phi Phi
