you'll need some luck to see dolphins!
Try the region between Istria and Cres or around Mali Losinj but I can't guarantee that you'll see them there. Depends on the sea. If there are too many waves you won't be able to see them...
I think it is better to look out for dolfins in the early or late season when there aren't too many people on the beachs. If you're on the sea it is important to be quiet because normally the dolfins are very curious and want to see what's in the water. So if you're patient enough they'll come to you!
Those boat-tours aren't recommendable and you can't be sure to see dolfins there...
Like others already have said you'll need a little luck. But don't waste your money with bout-tours that promise to show you dolfins!!