The short answer
Keep your plans and move them by an hour. Rain in Cartagena usually turns up as a heavy burst rather than a grey day, and the sky goes back to work fairly quickly afterwards. September and October are our wettest months, and we still run a full week of days out here through both of them.
What actually changes a day is wind, not water. A rainy morning with a flat sea is a normal day. A dry morning with a rough sea is the one where the logistics shift.
Whether the boat still goes
Our boats still leave Muelle de la Bodeguita at 10am, 11:30am and 5pm, and come back off the island at 4pm and 5:30pm. The crossing takes 25 minutes and is calmer than the Rosario Islands run, though in windy weather it gets choppy. It isn't white-knuckle, but if you hate boats you'll notice.
Here's the bit worth knowing before you book: if the tide is very high or the sea unusually rough, we don't use our own beach landing. We dock in Bocachica village instead, about a mile away, and you ride pillion on a motorbike taxi from the dock to our gate. In rain that is either the worst part of your day or the best story from your trip, and in our experience it's usually the second.
There's no phone signal on the island, so WhatsApp is how you check in with us. We answer reservations messages between 9am and 9pm, and a message at 8:30am on a doubtful-looking morning is the right move.
Wet plans that are actually better than the dry version
A dive. We're a PADI Dive Resort and our house reef is an eight minute swim off the beach, sitting at about six metres. Rain on the surface is irrelevant once you're under it. A one hour discovery dive needs no certificate, which makes it the single best use of an afternoon nobody can sunbathe through. This is also why we tell people not to bother snorkelling here: the reef is too deep and the surface too lively for it to be worth the mask.
Lunch that takes three hours. Our restaurant is open 10am to 11pm and most of it has a roof. Our Executive Chef is Pedro Mosqueda, who trained at the Gato Dumas Institute in Bogotá and cooked in leading Colombian kitchens before New York, and we were the first beach club in Cartagena to bother with a wine list. Rosé comes by the magnum. A downpour is a legitimate excuse to order like it's someone's birthday.
The ceviche class. Built for two, wine included, and it happens under cover. You learn one local dish properly instead of photographing eleven.
A massage, or yoga. Our wellness space does massage, yoga and personal training. Couples massages can be arranged on arrival or booked ahead. Ask us which, because it depends on the day.
Work, briefly. The Wi-Fi is high speed and holds a video call, and there are shaded tables near power sockets. Answering email while it hammers down outside, then swimming the moment it stops, is a defensible way to spend a Tuesday.
What not to do in the rain
Save the open-air things. Castillo San Felipe de Barajas and the city walls are the two Cartagena sights people most regret doing wet, because both are entirely exposed and the appeal is the view and the stonework rather than anything you can shelter under. They're still there tomorrow. Same for Bocachica village: our village tour starts with a five minute motorbike taxi and continues on foot to the San Fernando fortress, which is a fine morning in dry weather and a slog in a storm.
Mountain biking on the island trails is best early anyway, before the sun is fully up. Rain moves it from early to another day.
If you're stuck in the city
We won't publish museum opening hours we haven't checked this week, so ask your hotel to phone ahead rather than trusting a page. What we can tell you is that the Walled City is the right place to be caught out: the streets are narrow, the balconies overhang, and a coffee somewhere with a courtyard costs almost nothing. Cartagena is also very used to this. Nothing stops.
If the sea is genuinely too rough for the crossing and you'd booked a night with us, our sister property Townhouse sits in the Walled City, and residents staying more than a week are welcome to take the odd night there anyway.
Should you avoid the rainy season altogether?
No, and we'd say that even if we weren't the ones selling the boat ride. Rain arrives with fewer people, softer light and a sea that's often glassy between showers. If your trip is fixed to one perfect beach day and nothing else, book a week in the drier months. If you have three days and some flexibility about which one is the beach day, come whenever and move things around.
See you on the boat.