Outside seating on Catamarans and fast boats

We are coming to the islands around Split in May. We both do not like travelling inside boats as it makes us seasick. Is it possible to sit outside on the catamarans and other fast boats? Are the rules different between ferry companies and routes? If so, is there anywhere with the information in one place?

It mostly depend of type of ferry and company policy. Some ferries simply have no outside space, while others has plenty.
I would suggest you to choose your routes and check with specific company if they allow to sit outside on the catamarans and other fast boats.