Hi, it is true that ACI marina can be not too far away walking...
But if you are with luggage it can be a bit dodgy. The problem with the previous answer is that autobus number 8 is not close to where the bus from the airport will drop you...
Let me explain you like this: you walk down the bus. Enter the Riva (you cannot miss it, the sea will be on your left, the palace on your right. When the beautiful white floor is finished, you will see a bus station. That is where you can take number 8, that says Sv. Frane- ?njan.
I am not sure now how many stops you will need until you arrive to the marina, but probably just one or two. There is a big building with some glasses on it, they are working now on it. It is hotel Marjan, and more-less in front of it is the entrance to ACI Marina.