Hi Katja and Dan,
I hope that you enjoy your time on the Croatian Islands, which are naturally beautiful and historically fascinating. The water generally is very clear, with a visibility of 10 meters and more in most of the places where I went. Unfortunately, my quest for a freediving buddy was utterly fruitless. However, perhaps you two will have more luck, maybe together.
On Vis, I tried every dive shop and looked into every shred of advice from the locals. People were very friendly and some made phone calls for me. One person at the Issa dive center gave me Veljano Zanki's phone number, but he was not on the island, and when I called him he didn't want to be bothered to give me any advice. I was also told that someone at the Manta dive shop in Komiza might have been able to help me find a partner, but that didn't work out either. The divemaster who also freedives with Manta was very kind on the phone, but was booked solid with scuba diving excursions and didn't know anyone else who I could buddy up with. I wound up spending 10 days on Vis diving by myself, but staying well within my limits, going no deeper than 10 meters and spending no more than a minute under water.
I went to the island of Hvar afterward, where I had a good friend with a boat who took me to the Pakleni archipelago a few times. I did some spearfishing there with a harpoon (since you need a licence to use a speargun in Croatia). At this point, discouraged from my experience on Vis, I didn't even bother looking for a dive buddy. This is not to say, however, that I was entirely comfortable freediving by myself. I took a few precautions like taking some weight off of my belt, and surfacing after longer dives on an angle and in a position where my face would be out of the water should a syncope occur, but eventually I gave up such strategies as foolish. If anything happens, nothing beats having a partner on the surface. Staying well within my limits was probably the best thing I could do to dive alone safely.
Wishing you both pleasant and safe diving on the Croatian islands,
Raf