Mr. Tremac said that there were usually about 60 skippers online, waiting for customers, and that the goal was to have boats meet customers within 15 minutes of all requests. He would not say how often that goal had been met.

Recent figures show Croatia attracting 16 million tourists a year, mostly in July and August. There are regular ferry services to the most popular of its 1,200 islands, like Hvar and Brac, and a flotilla of boats for rent or for set tours can be found along the harbor front in Split and other coastal resorts.

UberBOAT is aimed at people who want a more personalized, on-demand service — and are willing to pay for it.

For example, a one-way Uber trip to Hvar from Split on an eight-seat boat cost 2,600 kuna (about $416, or $52 per person); ferry tickets run about $16. Uber Adventures cost a minimum of 330 kuna (about $53) plus $5.75 per kilometer and 50 cents per minute.

UberBOAT will also take visitors directly from Split’s waterfront airport to Hvar, about an hour’s journey, for about $480 for up to eight passengers, or $600 for up to 12. That would replace a half-hour $35 taxi ride from the airport to the line for the $16 ferry — where the wait for the next boat can be up to four hours and the water journey itself lasts two hours.

Mr. Tremac said the service was for people more concerned about saving time than money, noting that tourists spend an average of three and a half days in the area.