Getting to Olympia by  boat is common if your are traveler coming from Italy.

Ships travel between Italy and Greece on a daily basis and the options are many if you  wish to make this trip. The usual ports that are connected to Patras  are Brindisi, Bari and Ancona. All the ships arrive in Patras and from there all you have to do is use your own means of transport to travel towards Pyrgos, and then follow the road for 20 minutes and arrive in Olympia.
Alternatively, you may wish to use public transport (bus/train) and  that is very easy as there are many trains and buses leaving for  Pyrgos every day. Both the stations (bus, train) are within 5  minutes walking distance from the port were the ships dock.