The Plava Špilja sits on the open Adriatic side of the Luštica peninsula, about half an hour by speedboat from the Herceg Novi promenade. The cave is large enough for small boats to enter, and around midday sunlight reflecting off the white sand floor turns the whole interior an unreasonable shade of blue.
Most operators combine the cave with a stop at Žanjic and the abandoned submarine pens at Rose. Go in the morning before the wind picks up; the cave is too rough to enter on choppy days.