Hi Hardpole,
Sorry but it's 'D
ubrovnik' - not Debrovnik.

and sorry this is going to get long winded...
I'm from that neck of the woods and was there last for about 4 weeks in 2006, so don't know if any of this info is outdated, but considering how slowly the country progresses, and how laid back the people are - I doubt much has changed.
We didn't sail but drove the coast road all the way from Dubrovnik (south coast) to Pula (north coast) - its quite simply -
spectacular!
But my next trip will involve more boating/sailing for sure.
I think the windsurfing capital is a little town called 'Bol' - its on the island of 'Brac' (Brach).
It's where the 'Zlatni Rat' (Golden Cape) beach is - its the iconic beach on all the postcards.
You can get a ferry to the island from Split, but then its a car, bus or scooter ride to the other side of the island to the town.
The road into town is narrow, high and hairpin turns upon hairpin turns on the decent. On a scooter it would be quite scary, in a car, your passenger will suffer vertigo and sh1t themselves and beg you to slow down, I can't recall if all the bends had barriers either - probably not. BUT its an awesome view! (and a long way down - so try stay on the road). ...If you go on a bus - keep your eyes closed and pray.

The stretch of water between the island Brac (town Bol) and the island of Hvar (about 4-5km) is windy most afternoons and famous for Croatia's windsurfing scene - probably more slalom than anything else.
All through the Adriatic - especially between the islands there is very little tidal movement and practically no ocean swell (heaps of islands) - so any lumps are driven purely by wind.
Due to the geography (high altitude mountain ranges right next to the sea) the region gets all sorts of consistent seasonal winds, and each direction of wind has its own specific name - none of this NW, SE, southerly etc. think 'the Fremantle Doctor'. There is a bunch of them, but during May you should be getting the 'Maestral' (westerly from noon to dusk, 15-30knots maybe).
Just out of the town, towards the cape, and just before the spit is a little windsurf hire place with instructors, they speak English and were friendly. I've got no idea what sort of equipment they would have, but I'm sure it would be decent these days. Its all within walking distance.
If you are yachting from Split to Dubrovnik, then you might be stopping at Bol on the way through, I'd be surprised if that wasn't already on your sailing itinerary as its such an iconic stop.
If you decide to stay in Bol, the little town is beautiful, accommodation is easy to find. There's a tourist info centre right next to the marina - go there first and ask them to hook you up - you'll get good, cheap accommodation at someone's apartment/house - I'd tend to avoid the big hotels and resorts - not that there was many in Bol (they can be overpriced). Plenty of the locals on the coast run apartments/townhouses B&B's etc privately - better value for money and you might get some good home cooking or home made wine or plum or grape brandy "Rakija" (handle with care).
You could quite easily spend a week in Bol and you won't regret it, you might not want to come back! If you have a better have - they'll love it too!
I'll see if I can find out and send you some more info, I'll upload some pics too.
Cheers.
PS: you probably want to know what the windsurfing was like?...
Actually, I didn't do any!
When you get to the beach down there, and you see all these young beautiful women in bikinis (and some without bikinis), AND you can take your beer/cocktail from the beach bar right down to your towel or beach chair on the actual beach - you think to yourself... windsurfing can wait till tomorrow!

...3 days later, still no windsurfing and its time to go.