Getting from the airport to Athens city center Piraeus and many of the suburbs is very easy in many ways. If you do have time to spend and are used to the public transportation you can use the Metro and even a combination with the Tram. If your time is limited there are taxis available and minibuses that have a quite low cost (taxi fares start from 35 euro for a ride from the airport to the city center).
If your final destination from the airport is Alimos marina you can take the X96 bus that will drive you exactly outside the marina and your walking distance after will not be longer than 15 minutes. Make sure that you know the exact location of your yacht as there are three bus stops along the marina.