Blue Cave Montenegro, from Kotor

Next comes the islet of Our Lady of the Rocks, where you can step ashore for 20 minutes, then a 25-minute run to a former submarine tunnel for a 10-minute visit. After that the boat heads for the mouth of the bay, passes the round fortress of Mamula and makes the last 10-minute dash to the Blue Cave.

The swim at the cave is optional and lasts 30 minutes, then the boat brings you back to one of two drop-off points. A licensed captain drives and a licensed local guide tells the stories, with music on board and an audio guide in 8 languages on your own phone.

On sunny summer days, and especially before noon, sunlight bounces off the sea and in through the smaller opening, colouring the water and the vault a deep blue. The national tourism organisation says the water is so blue that the cave is called the Sapphire of Boka.

The chamber is small and round, shaped by the sea. From the water to the top of the vault it is about 9 m, the sea inside is 3 to 4 m deep, and the cave covers about 100 square metres at sea level. It takes a minute to cross and much longer to stop looking down.

The ride gets its own comments. Guests praise the music on board and the speed, and some warn that the front seats are bumpy, so sit further back if you prefer a smoother crossing. The tour is rated 4.86 from 4,443 reviews.

The sights earn their share of praise too: guests single out the Blue Cave, Our Lady of the Rocks and the submarine tunnels. Several reviewers also call the tour good value for the money.

High summer is the brightest and driest time. At the coastal station of Bar the average high reaches 30.7 °C in July and 31 °C in August, and August sees on average a single wet day of 2.5 mm or more. June and September are a little cooler.

The cave glows best on sunny mornings, so a morning departure gives you the best chance of the deep blue. The tour runs rain or shine, with raincoats handed out when it rains.

That is also why the free cancellation matters. Until 24 hours before departure a cancellation is refunded in full, and a booking can be made now and paid later. Watch the forecast a couple of days ahead and move your date while you still can.

If you are in the area for several days, book early in your stay. A spare morning later in the week turns a weather cancellation into a delay instead of a missed trip.

The ministry acted after several speedboats had entered the cave at once, boats had come too close to swimmers and some skippers had gone in during bad weather and struggled to get out. The rule answers exactly those problems.

Snorkeling masks are on board when available, along with life jackets. Wear your swimsuit under your clothes from the start and bring a towel. The swim is optional, so anyone who prefers to stay dry can enjoy the colour from the boat.

Inside the church hang sixty-eight paintings, most by the Perast-born Baroque painter Tripo Kokolja, and about 2,000 silver votive plates. The islet admission is not part of the tour price and is paid in cash, per guest.

Swimwear is not allowed for the visit: cover your shoulders and wear modest shorts, a skirt or a dress. Keep your swimsuit for later, under your clothes.

A little further on, the boat passes Mamula, a round Austrian fortress about 200 m across on a small island at the entrance to the bay. From March 1942 to September 1943 it was an Italian internment camp, and more than 1,500 people passed through it.

Today the fortress is a hotel, categorised 5-star in June 2023. The tour passes it for about 5 minutes on the way to the cave.

Bring a towel, sun cream, a hat and a dry layer for the ride back, which is fast and breezy. A waterproof pouch keeps your phone safe, and your phone with mobile data also plays the audio guide.

Carry some cash in small notes for the islet admission and, if you wish, a tip for the crew; neither is included. Arrive 30 minutes before the start for check-in, because a late arrival can lose the slot without a refund.

For the view without the climb, the cable car runs from Dub near Kotor to Kuk village on Mount Lovćen, at 1,348 m. The ride takes 11 minutes and covers a route of 3,900 m.

The port lies 100 m from the old town, which makes a morning boat tour easy to fit around a cruise ship's call. Leave enough time between the return and your ship's all-aboard.

It suits less well if you want long stops. The islet visit is 20 minutes and the tunnel 10, and Perast is seen only from the water. If walking through Perast matters, look at our comparison of the other boats.

And remember that the cave depends on the sea. On a rough day the route changes, so keep your plans flexible and book with free cancellation.