The most appropriate mode of transport for you trip will depend on the country you are travelling to and the locations of the gardens you intend to visit - so it is important to plan your transportation before you set off.
Ferry timetables tend to be complicated and variable but very good online information is available (eg on Mediterranean ferries). Advance reservations are necessary for vehicles but pedestrians can almost always arrive at the port and get a good deal on the ferry ticket, perhaps taking advantage of special offers or seasonal changes in ferry ticket prices.
For Greek Ferries see Greek Ferries
For Channel Ferries see Ferry Booker
For European Ferries see Europe Ferry Crossings for bookings.