Patras marina is on left side of Patras port ( when you come from sea). First pontone is for transit visitors. You have water and electricity on the pontone and price is very low 29 EURos for 3 days ( if you wont to stay 4 days you will pay for 6 days). Every day at the evening you can buy fuel from lorry which is parked near mole. Mole is private and owner is very unpolite - but gentlemen who works on pontone will help you in any way. 10 min walk around is center of Patras and very good market for provisions. I recomend visit Ancient Olympia by bus as it is perfect one day visit. Enjoy!
I was there in 1998 and found the visitor-berth very poor when it blows. In fact a number of boats had problems due to the waves coming into the marina. The first pontoon, where the visitors-berth are is by far the worst. I seem to recall that this is also mentioned in the pilot.