A cheap ferry for pedestrians to Tavshanli costs about the same as a city ferry in Istanbul (about $0.7). It mainly transports cars and trucks, which are more expensive for them, of course. From Marmaray Gebze, you can take the 416 bus, but it's a bit difficult to get to the stop (from the first car from the center, go right, then straight down the street to the bridge, turn right again at the bridge, and then cross the street to the left at the first lane there and stomp to the stop where this bus is the only one that stops). Or by taxi. Tickets are purchased on the spot, you do not need to book anything in advance. Ships depart on average every 20-25 minutes, travel time is about the same, navigation is around the clock. There is a minibus from the marina in Tavshanli to Yalova