You will need to research the ferry and catamaran timetables, but services are more limited in early May. The ferry goes to Stari Grad and takes 2 hours each way, whereas the catamaran takes an hour and goes into Hvar town. I seem to recall that a day trip via the cat isn't possible at that time of year. The cat is more likely to be cancelled in bad weather than the ferry, but both should be ok by May.
Some places in the bigger towns will be open, but it is very early in the season, so not everything will be up and running. Families that live on Hvar for most of the year often spend December through to the end of April on the mainland.