Sorry for the late reply, but if you don't mind I'll share my experiences too.
I was scuba diving in Croatia for a week in the end of September this year. My girlfriend and I went to the island Rab and we staied in a small town called Lopar. Since it's really small it only has one dive center, so we didn't have much choice, but I don't regret not even for a second choosing Moby Dick diving center ( www mobydick-diving com ). It's owned by this really nice couple Mladen and Sandra. We where diving 5 days, made 10 dives and not even once on the same location. Mladen took us to verious locations around island Rab (like island Goli, Grgur, Prvič...) by his boat.
We dived with 5mm and 6,5mm weat suits and I can say it wasn't too cold.
Also we had a huge luck with the weather since it was raining for a week before we got there, then it was sunny and warm the whole week, water got warmer then 20°C and about 15°C on 25m, and then on the last day it weather took a turn to worse and after we left there was another week of rain
Last weekend we had an ending of Nitrox course and we ended it with a diving day in Kostrena (near Rijeka). We only made 2 dives but they were really nice, beautiful location full of marine life. Water had 15°C on top and 20°C on about 35m. ( www dckostrena hr )
And one more usefull thing for all of you who are considering going to Croatia to do some diving. Here is a link with 116 most beautiful diving locations in Croatia also listed all diving centers in the country, a map with locations and the discriptions of the locatons: www ronjenjehrvatska com/en/
PS: Since I'm new on this forum I'm not allowed to post links just yet, so wherever you see links divided into 3 parts just puzzle them together
