We go to Bol on the island Brac, just south of Split (
http:///www.bol-croatia.com/) We dive with Big Blue in Bol (south of the island) twice a year. We get the bus from the airport to Split port (from memory about £3, 35 mins) and get the daily 1600 ferry direct to Bol (c.£2.00 single trip). (
Jadrolinija Ancona - Split Ancona Zadar Dubrovnik - Bari.) It returns to Split at 0630 each day in season (Sundays maybe 0730?)
There are other ferries during the day to Supetar (north of the island) and then bus/taxi to Bol.
Can't tell you about the other islands but the ferries tend to have a circuit to other ports/islands.
We take 2 sets of wetsuit gear inc. BCs, fins etc and whilst I wouldn't do it every day it's OK (it weighed in at Split Airport last time at 33kgs!). The port area of Split is generally flat, with loads of eating/buying places. Check out the market (next to port area) in the mornings. Both dive centres in Bol are 10 min walk on good flat surfaces, or taxi maybe £5?. If you stay in Bol, a lot of it is hilly, be warned!
We go for a week and dive twice a day, meet at 0930, back around 1230 and out again at 1400 - all 'small boat dives' with maybe 6-8 divers on average, sometimes only 4, but it can get very busy in July/August. Nice diving from 12-50m, great viz, not as diverse or as colourful as the Red Sea but not as busy or 'organised' either.
try this video from Bol last year for an idea
YouTube - Scuba diving on Island Hvar, Croatia. September 2007
For more general info try this thread from last year.
http://www.scubaboard.com/forums/central-eastern-europe/201087-general-diving-info-croatia.html
Cheers
Adrian