Hon Mun is the reason the boats exist. It is also the first thing the sea takes away. Between October and November the swell in Nha Trang bay builds to the point where the crossing to the outer islands becomes genuinely unsafe, and, according to Alma Resort and Local Vietnam, those two months carry the highest number of cancelled boat days in the year. What happens next is worth understanding before you book a winter trip, because it is not a shutdown - it is a redirection.
The short version. November: around 330 mm of rain, the wettest month. February: about 49 mm, the driest. April to August: the clean window for snorkelling and diving around Hon Mun. October-November: the peak of cancelled outer-island departures.
What the weather actually does
The rainfall figures split the year cleanly. November, with roughly 330 mm, is the wettest month on the coast; February, at around 49 mm, is the driest; January to March is described as cool and dry with minimal rain.
But rainfall is not the operative variable for a boat - swell is. The outer reef sits 40-50 minutes from shore with nothing between it and open water, and that exposure is what closes it, days before any rain does.
Where do the boats go instead?
Read the fine print of three unrelated Nha Trang operators and the same instinct appears. Vietnam Fantastic writes into its own programme note that the schedule changes with the real weather situation, and that in bad weather the boat runs to the coral reef instead of Mun Island. Vuongland lists "speed boat / wooden boat (depend the weather)" inside its inclusions - the vessel changes, not the day. VictorDivePro says the route is chosen by the weather and the visibility of that day.
None of the three publishes a winter timetable, and none of them promises the outer reef unconditionally. That is the honest reading: on a rough day you get a different point, usually inside the bay.
The charter rate card shows what that substitution costs. A speedboat to Hon Mun is 1,900,000-4,900,000 VND (about $73-188) for the boat; the same operator's line to Hon Tam is 1,400,000-3,900,000 VND (about $54-150) and to Vinh San Ho 1,200,000-3,200,000 VND (about $46-123). Moving into shelter is also the cheaper column.
Who says something about the season out loud
Most operators here publish nothing at all about winter. Two are exceptions, in opposite directions.
Sailing Club Divers writes that it runs trips to Madonna Rock, Dam Bay, Lobster Cave, the Ship Wreck, Big Wall, Cemetery and the Hidden Wall every day throughout the year, and prices one day with three dives at 100 USD on its product page. Of the Nha Trang operators with a published price list, Sailing Club Divers is the only one that claims year-round work.
The Kiterabbit school goes the other way and simply leaves: from November to March it moves kitesurf teaching to Phan Rang, a knee-deep lagoon down the coast, which the school says halves the learning time. In Nha Trang itself it keeps the surf lessons at $65 for a two-hour private session, with the plain note that whether a lesson happens depends on conditions.
The sheltered-water options
There is a whole class of activity in Nha Trang that never needed the outer reef. The FlyGuru eFoil school charges 2,000,000 VND (about $77) for a 60-minute beginner session and works off Maryna Beach Club, which the operator describes as a safe bay without waves or currents - a statement about the place rather than about the season, but the place is the point.
South of the airport, the bay of Cam Ranh is a second body of sheltered water. The boat-and-SUP outing there costs 490,000 VND (about $19) per guest for three to four hours, and its operator, Cam Ranh Boat Trip, is the only one among the Nha Trang and Cam Ranh operators with a published price list to print a bad-weather rule: if the trip cannot run, the deposit is refunded or the date is moved.
What should you ask before you pay?
Three questions, in this order. Which point is the alternative if Hon Mun is closed that day? Does the vessel change, and to what? And what happens to the money - because the answers differ sharply: the 600,000 VND (about $23) Vuongland day charges the full price for a cancellation inside twenty-four hours, while the Cam Ranh operator refunds or reschedules.
Is it worth going out at all in December?
Yes, but book the inner bays rather than the reef, and treat the outer islands as a bonus if the day is calm.
When is the reef reliably open?
April to August is the window the climate data points to for snorkelling and diving around Hon Mun.
Does the rain ruin the day?
Rain alone rarely does; swell does. A wet, flat morning is a workable boat day, a dry windy one often is not.
Updated 4 September 2026. Rainfall from published climate summaries for Nha Trang, cancellation patterns from Alma Resort and Local Vietnam; operator statements and prices from the operators' own pages.
