Boat taxis are a genuinely useful and fun way to reach certain beaches that are awkward to get to by road. From Herceg Novi and the Bay of Kotor, small water taxis run to beaches on the Lustica Peninsula such as Zanjic and Mirista — two gorgeous clear-water pebble coves that would otherwise require a long drive and a steep walk.
In season (roughly June through September) these boats operate informally from the main waterfront promenades; you'll usually see signs or touts near the harbour. Prices vary by distance and negotiation, but short hops typically cost a few euros per person each way.
For Rose village and some smaller Boka Bay coves, similar informal water taxis operate from nearby towns. It's worth asking at your accommodation or the local tourist office for the most up-to-date options, as services depend on demand and weather.