Lokrum island is really a monestary. Now there is a botanical gardens, some walks and some Game of Thrones themed nonsense.

At one end of the island not too far from the port is a small lagoon that is sometimes connected to the sea through a channel and sometimes not.
The water in it is 'half fresh' as a result so tasted only very slightly salty.

Photos below show the lake, the cave inlet where it is connected to the sea through a rock passage and the coast on the other side.
I love the fact that there are places in the world where they don't nanny you. The cliff was completely open and while I was there someone's five year old child was scrambling over the rocks having fun a few feet from sudden death with no barriers whatsoever. But since they hadn't been brought up to assume everything everywhere was safe, they were probably fine although I can imagine American parents having kittens.