There are two interesting places in Montenegro that can be visited only by water. They are the Blue Cave and Mamula island.
In 2014 a horror-fantasy film Mamula was made, it is about this island.
The island has a different name, but everyone knows it because of the Mamula Fort. The fort was named after the Austrian general who built it in the 19th century. It was used as a fort, later as a prison, took part in both world wars, but after the World War II it's abandoned. There are no people on the island, so you can enter the fort for free. Despite the fact that it looks like new and is well preserved, always be careful and watch your step.
The Blue Cave is one of many caves at this location, but it's the only one that you can visit even on a boat, so it has become a tourist spot. When your eyes adjust to the darkness, you will see why the cave is called blue. Even better it's noticeable under the water (see the photo below). You can swim in the cave.
During the trip you will watch the beautiful sea views.
Getting there: The easiest way to get to the Mamula island and Blue Cave is by boat from the Zanjic beach. We arrived at the beach near 5 pm and found out that the boats stopped working, so the next day we went there from Herceg Novi. The boat from there is much more expensive (80-100 euros) and goes longer, but it's worth it. Tell the boat driver that you want to visit both the island and the cave.